How can I prevent SQL injection in PHP?
1
If user input is inserted into an SQL query directly, the application becomes vulnerable to SQL injection, like in the following example:

That's because the user can input something like value'); DROP TABLE table;--, making the query:

What should one do to prevent this?
improve this question | comment
Aileen MacGyver Created at: 2013-11-13 17:07:52 UTC By Aileen MacGyver
Note that that particular example will not work, because the mysql_ lib does not allow executing 2 queries in one statement. As long as you keep the manipulation in a single statement the injection will work. - Earl Schmidt
It also shouldn't work if you have given proper permissions to the user and assuming that the average sql user should not be granted permissions to drop tables it shouldn't work. - Giuseppe Hamill
Please, don't use mysql_* functions in new code. They are no longer maintained and are officially deprecated. See the red box? Learn about prepared statements instead, and use PDO or MySQLi - this article will help you decide which. If you choose PDO, here is a good tutorial. - Princess Oberbrunner
SQL Injection ---> php.net/manual/en/security.database.sql-injection.php - Roberto Lang
@vlzvl m_r_e_s() is never sufficient. Proof of concept where it fails: ilia.ws/archives/… - Carleton Padberg
27 Answers
0
use mysql_real_escape_string follow link: http://php.net/manual/en/function.mysql-real-escape-string.php
0
The right way to prevent SQL injection is by using parameterized queries. This means defining the SQL code that is to be executed with placeholders for parameter values, programmatically adding the parameter values, then executing the query. Doing this allows the server to create an execution plan for the query, which prevents any "injected" SQL from being executed. An example will help in explaining this. Let’s use the same script, but I’ll define the SQL query with parameter placeholders:

$sql = "SELECT * FROM Tbl WHERE Username = ? and Password = ?";


Now, I’ll define an array that holds the parameter values:

$params = array($_POST['Username'], $_POST['Password']);


When I execute the query, I pass the $params array as an argument:

$stmt = sqlsrv_query($conn, $sql, $params);

0
Use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.

You basically have two options to achieve this:

Using PDO:

$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');

$stmt->execute(array('name' => $name));

foreach ($stmt as $row) {
    // do something with $row
}

Using mysqli:

$stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
$stmt->bind_param('s', $name);

$stmt->execute();

$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    // do something with $row
}

PDO

Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:

$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8', 'user', 'pass');

$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);


In the above example the error mode isn't strictly necessary, but it is advised to add it. This way the script will not stop with a Fatal Error when something goes wrong. And gives the developer the chance to catch any error(s) which are thrown as PDOExceptions.

What is mandatory however is the first setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it to the MySQL server (giving a possible attacker no chance to inject malicious SQL).

Although you can set the charset in the options of the constructor it's important to note that 'older' versions of PHP (< 5.3.6) silently ignored the charset parameter in the DSN.

Explanation

What happens is that the SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute the prepared statement is combined with the parameter values you specify. 

The important thing here is that the parameter values are combined with the compiled statement, not a SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters you limit the risk of ending up with something you didn't intend. Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains 'Sarah'; DELETE * FROM employees the result would simply be a search for the string "'Sarah'; DELETE * FROM employees", and you will not end up with an empty table.

Another benefit with using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.

Oh, and since you asked about how to do it for an insert, here's an example (using PDO):

$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');

$preparedStatement->execute(array('column' => $unsafeValue));

0
Using this PHP function mysql_escape_string() you can get a good prevention in a fast way.

For example: 

SELECT * FROM users WHERE name = '".mysql_escape_string($name_from_html_form)."'


mysql_escape_string — Escapes a string for use in a mysql_query

For more prevention you can add at the end ... 

wHERE 1=1   or  LIMIT 1


Finally you get:

SELECT * FROM users WHERE name = '".mysql_escape_string($name_from_html_form)."' LIMIT 1

0
I use three different ways to prevent my web application from being vulnerable to SQL injection.

Use of mysql_real_escape_string(), which is a pre-defined function in PHP, and this code add backslashes to the following characters: \x00, \n, \r, \, ', " and \x1a. Pass the input values as parameters to minimize the chance of SQL injection.
Use of MySQLi.
The most advanced way is to use PDOs.
I hope this will help you.
0
You've got two options - escaping the special characters in your unsafe_variable, or using a parameterized query. Both would protect you from SQL injection. The parameterized query is considered the better practice, but escaping characters in your variable will require fewer changes.

We'll do the simpler string escaping one first.

//Connect

$unsafe_variable = $_POST["user-input"]
$safe_variable = mysql_real_escape_string($unsafe_variable);

mysql_query("INSERT INTO table (column) VALUES ('" . $safe_variable . "')");

//Disconnect


See also, the details of the mysql_real_escape_string function.

To use the parameterized query, you need to use MySQLi rather than the MySQL functions. To rewrite your example, we would need something like the following.

<?php
    $mysqli = new mysqli("server", "username", "password", "database_name");

    // TODO - Check that connection was successful.

    $unsafe_variable = $_POST["user-input"];

    $stmt = $mysqli->prepare("INSERT INTO table (column) VALUES (?)");

    // TODO check that $stmt creation succeeded

    // "s" means the database expects a string
    $stmt->bind_param("s", $unsafe_variable);

    $stmt->execute();

    $stmt->close();

    $mysqli->close();
?>


The key function you'll want to read up on there would be mysqli::prepare.

Also, as others have suggested, you may find it useful/easier to step up a layer of abstraction with something like PDO.

Please note that the case you asked about is a fairly simple one, and that more complex cases may require more complex approaches. In particular:

If you want to alter the structure of the SQL based on user input, parameterised queries are not going to help, and the escaping required is not covered by mysql_real_escape_string. In this kind of case you would be better off passing the user's input through a whitelist to ensure only 'safe' values are allowed through.
If you use integers from user input in a condition and take the mysql_real_escape_string approach, you will suffer from the problem described by Polynomial in the comments below. This case is trickier because integers would not be surrounded by quotes, so you could deal with by validating that the user input contains only digits.
There are likely other cases I'm not aware of. You might find http://webappsec.org/projects/articles/091007.txt a useful resource on some of the more subtle problems you can encounter.
0
I'd recommend using PDO (PHP Data Objects) to run parameterized SQL queries.  Not only does this protect against SQL injection, it also speeds up queries.  And by using PDO rather than mysql_, mysqli_, and pgsql_ functions, you make your app a little more abstracted from the database, in the rare occurence that you have to switch database providers.
0
Every answer here covers only part of the problem.
In fact, there are four different query parts which we can add to it dynamically:

a string
a number
an identifier
a syntax keyword.
and prepared statements covers only 2 of them

But sometimes we have to make our query even more dynamic, adding operators or identifiers as well.
So, we will need different protection techniques.

In general, such a protection approach is based on whitelisting.
In this case every dynamic parameter should be hardcoded in your script and chosen from that set.
For example, to do dynamic ordering:

$orders  = array("name","price","qty"); //field names
$key     = array_search($_GET['sort'],$orders)); // see if we have such a name
$orderby = $orders[$key]; //if not, first one will be set automatically. smart enuf :)
$query   = "SELECT * FROM `table` ORDER BY $orderby"; //value is safe


However, there is another way to secure identifiers - escaping. As long as you have an identifier quoted, you can escape backticks inside by doubling them. 

As a further step we can borrow a truly brilliant idea of using some placeholder (a proxy to represent the actual value in the query) from the prepared statements and invent a placeholder of another type - an identifier placeholder.

So, to make long story short: it's a placeholder, not prepared statement can be considered as a silver bullet.  

So, a general recommendation may be phrased asAs long as you are adding dynamic parts to the query using placeholders (and these placeholders properly processed of course), you can be sure that your query is safe.

Still there is an issue with SQL syntax keywords (such as AND, DESC and such) but whitelisting seems the only approach in this case.
0
Use PDO and prepared queries.

($conn is a PDO object)

$stmt = $conn->prepare("INSERT INTO tbl VALUES(:id, :name)");
$stmt->bindValue(':id', $id);
$stmt->bindValue(':name', $name);
$stmt->execute();

0
Whatever you do end up using, make sure that you check your input hasn't already been mangled by magic_quotes or some other well-meaning rubbish, and if necessary, run it through stripslashes or whatever to sanitise it.
0
Injection Prevention - mysql_real_escape_string()

PHP has a specially-made function to prevent these attacks. All you need to do is use the mouthful of a function mysql_real_escape_string.

What mysql_real_escape_string does is take a string that is going to be used in a MySQL query and return the same string with all SQL Injection attempts safely escaped. Basically, it will replace those troublesome quotes(') a user might enter with a MySQL-safe substitute, an escaped quote \'.

NOTE: you must be connected to the database to use this function!

// connect to MySQL

$name_bad = "' OR 1'"; 

$name_bad = mysql_real_escape_string($name_bad);

$query_bad = "SELECT * FROM customers WHERE username = '$name_bad'";
echo "Escaped Bad Injection: <br />" . $query_bad . "<br />";


$name_evil = "'; DELETE FROM customers WHERE 1 or username = '"; 

$name_evil = mysql_real_escape_string($name_evil);

$query_evil = "SELECT * FROM customers WHERE username = '$name_evil'";
echo "Escaped Evil Injection: <br />" . $query_evil;


you can find more detail about SQL Injection Prevention. 
0
You could do something basic like this:

$safe_variable = mysql_real_escape_string($_POST["user-input"]);
mysql_query("INSERT INTO table (column) VALUES ('" . $safe_variable . "')");


This won't solve every problem, but it's a very good stepping stone. I left out obvious items such as checking the variable's existence, format (numbers, letters, etc.).
0
As you can see, people suggest you to use prepared statements at the most. Its not wrong, but when your query is executed just once per process, there would be a slightly performance penalty. 

I was facing this issue but I think i solved it in very sophisticated way - the way hackers use to avoid using quotes. I used this in conjuction with emulated prepared statements. I use it to prevent all kinds of possible sql injection attacks.

My approach:

if you expect input to be integer make sure its really integer. In variable-type language like is php is this very important. You can use for example this very simple but powerful solution: sprintf("SELECT 1,2,3 FROM table WHERE 4 = %u", $input);  
if you expect anything else from integer hex it. If you hex it, you will perfectly escape all input. In C/C++ there's a function called mysql_hex_string(), in php you can use bin2hex().

Dont worry about that the escaped string will have 2x size of its original length because even if you use mysql_real_escape_string, php has to allocate same capacity ((2*input_length)+1), which is the same.
This hex method is often used when you transfer binary data but I see no reason why not use it to all data to prevent sql injection attacks. Note that you have to prepend data with 0x or use mysql function UNHEX instead.
So for example query:

SELECT password FROM users WHERE name = 'root'


Will become:

SELECT password FROM users WHERE name = 0x726f6f74


or

SELECT password FROM users WHERE name = UNHEX('726f6f74')


Hex is the perfect escape. No way to inject.

Difference between UNHEX function and 0x prefix

There was some discussion in comments, so I finally want to make it clear. These two approaches are very similar but are a little different in some ways:

0x prefix can be only used on data columns such as char, varchar, text, block, binary, etc.
Also its use is a little complicated if you are about to insert an empty string, you'll have to entirely replace it with '', or you'll get an error.

UNHEX() works on any column; you do not have to worry about the empty string.

Hex methods are often used as Attacks

Note that this hex method is often used as a sql injection attack where integers are just like strings and escaped just with mysql_real_escape_string, then you can avoid use of quotes.

For example if you just do something like this:

"SELECT title FROM article WHERE id = " . mysql_real_escape_string($_GET["id"])


attack can inject you very easily. Consider the following injected code returned from your script:


  SELECT ... WHERE id = -1 union all select table_name from information_schema.tables


and now just extract table structure:


  SELECT ... WHERE id = -1 union all select column_name from information_schema.column where table_name = 0x61727469636c65


and then just select whatever data ones want. Cool isn't it ?

But if would the coder of injectable site hex it, no injection would be possible because query would look like this: SELECT ... WHERE id = UNHEX('2d312075...3635')
0
Type cast if possible your parameters. But it's only working on simple types like int, bool and float.

$unsafe_variable = $_POST['user_id'];

$safe_variable = (int)$unsafe_variable ;

mysql_query("INSERT INTO table (column) VALUES ('" . $safe_variable . "')");

0
In my opinion, the best way to generally prevent SQL injection in your PHP app (or any web app, for that matter) is to think about your application's architecture. If the only way to protect against SQL injection is to remember to use a special method or function that does The Right Thing every time you talk to the database, you are doing it wrong. That way, it's just a matter of time until you forget to correctly format your query at some point in your code.

Adopting the MVC pattern and a framework like CakePHP or CodeIgniter is probably the right way to go: Common tasks like creating secure database queries have been solved and centrally implemented in such frameworks. They help you to organize your web app in a sensible way and make you think more about loading and saving objects than about securely constructing single SQL queries. 
0
I favor stored procedures (mySQL has sp support since 5.0) from a security point of view - the advantages are -

Most databases (including mySQL) enable user access to be restricted to executing stored procedures. The fine grained security access control is useful to prevent escalation of privileges attacks. This prevents compromised applications from being able to run SQL directly against the database.
They abstract the raw SQL query from the application so less information of the db structure is available to the application. This makes it harder or people to understand the underlying structure of the database and design suitable attacks.
They accept only parameters so the advantages of parameterized queries are there. of course - IMO you still need to sanitize your input - especially if you are using dynamic SQL inside the stored procedure.
The disadvantages are -

They (stored procedures) are tough to maintain and tend to multiply very quickly. This makes managing them an issue.
They are not very suitable for dynamic queries - if they are built to accept dynamic code as parameters then a lot of the advantages are negated.
0
There are many ways of preventing SQL injections and other SQL hacks. You can easily find it on the Internet (Google Search). Of course PDO is one of the good solution. But I would like to suggest you some good links prevention from SQL Injection.

What is SQL injection and how to prevent

PHP manual for SQL injection

Microsoft explanation of SQL injection and prevention in PHP

and some other like Preventing SQL injection with MySQL and PHP

Now, why you do you need to prevent your query from SQL injection?

I would like to let you know: Why do we try for preventing SQL injection with a short example below:

Query for login authentication match:

$query="select * from users where email='".$_POST['email']."' and password='".$_POST['password']."' ";


Now, if someone (a hacker) puts

$_POST['email']= admin@emali.com' OR '1=1


and password anything....

The query will be parsed in the system only upto:

$query="select * from users where email='admin@emali.com' OR '1=1';


The other part will be discarded. So, what will happen? A non-authorized user (hacker) will be able to login as admin without having his password. Now, he can do anything what admin/email person can do. See, it's very dangerous if SQL injection is not prevented.
0
For those unsure of how to use PDO (coming from the mysql_ functions), I made a very, very simple PDO wrapper that is a single file. It exists to show how easy it is to do all the common things applications need done. Works with PostgreSQL, MySQL, and SQLite.

Basically, read it while you read the manual to see how to put the PDO functions to use in real life to make it simple to store and retrieve values in the format you want.


  I want a single column

$count = DB::column('SELECT COUNT(*) FROM `user`);

  
  I want an array(key => value) results (i.e. for making a selectbox)

$pairs = DB::pairs('SELECT `id`, `username` FROM `user`);

  
  I want a single row result

$user = DB::row('SELECT * FROM `user` WHERE `id` = ?', array($user_id));

  
  I want an array of results

$banned_users = DB::fetch('SELECT * FROM `user` WHERE `banned` = ?', array(TRUE));


0
If you want to take advantage of cache engines like redis or memcache maybe DALMP could be a choice, it uses pure mysqli, check this: http://www.dalmp.com/PExecute 

Also you can 'prepare' your arguments before preparing your query so that you can build dynamic queries and at the end have a full prepared statements query. http://www.dalmp.com/Prepare
0
I think if someone want use PHP and MySQL or other DataBase Server: 

Think about learning PDO (PHP Data Objects) – is a database access layer providing a uniform method of access to multiple databases.
Think about learning mysqli
Use native php functions like: strip_tags, mysql_real_escape_string or if variable numeric just (int)$foo. Read more about type of variables in PHP read here. If you using libs such as PDO or MySQLi always use PDO::quote() and mysqli_real_escape_string().
Libraries examples:

---- PDO


  ----- no placeholders - ripe for SQL Injection! It's bad


$request = $pdoConnection->("INSERT INTO parents (name, addr, city) values ($name, $addr, $city)");



  ----- unnamed placeholders  


$request = $pdoConnection->("INSERT INTO parents (name, addr, city) values (?, ?, ?);



  ----- named placeholders 


$request = $pdoConnection->("INSERT INTO parents (name, addr, city) value (:name, :addr, :city)");


--- MySQLi

$request = $mysqliConnection->prepare('
       SELECT * FROM trainers
       WHERE name = ?
       AND email = ?
       AND last_login > ?');

    $query->bind_param('first_param', 'second_param', $mail, time() - 3600);
    $query->execute();


P.S


  PDO wins this battle with ease. With support for twelve
  different database drivers  and named parameters, we can ignore the
  small performance loss, and get used to its API. From a security
  standpoint, both of them are safe as long as the developer uses them
  the way they are supposed to be used
  
  But while both PDO and MySQLi are quite fast, MySQLi performs
  insignificantly faster in benchmarks – ~2.5% for non-prepared
  statements, and ~6.5% for prepared ones.
  
  And please test every query to your database - it's better way to prevent injection 

0
The simple alternative to this problem could be solved by granting appropriate permissions in the database itself.
For example: if you are using mysql database. then enter into the database through terminal or the ui provided and just follow this command:

 GRANT SELECT, INSERT, DELETE ON database TO username@'localhost' IDENTIFIED BY 'password';


This will restrict the user to only get confined with the specified query's only. Remove the delete permission and so the data would never get deleted from the query fired from the php page.
The second thing to do is to flush the privileges so that the mysql refreshes the permissions and updates.

FLUSH PRIVILEGES; 


more information about flush.

To see the current privileges for the user fire the following query.

    select * from mysql.user where User='username';


Learn more about GRANT.
0
Regarding to many useful answers, I hope to add some values to this thread.
SQL injection is type of attack that can be done through user inputs (Inputs that filled by user and then used inside queries), The SQL injection patterns are correct query syntax while we can call it: bad queries for bad reasons, we assume that there might be bad person that try to get secret information (by passing access control) that affect the three principles of security (Confidentiality, Integrity, Availability). 

Now, our point is to prevent security threats such as SQL injection attacks, the question asking (How to prevent SQL injection attack using PHP), be more realistic, data filtering or clearing input data is the case when using user-input data inside such query, using PHP or any other programming language is not the case, or as recommended by more people to use modern technology such as prepared statement or any other tools that currently supporting SQL injection prevention, consider that these tools not available anymore? how you secure your application?

My approach against SQL injection is: clearing user-input data before sending it to database (before using it inside any query).

Data filtering for (Converting unsafe data to safe data)
Consider that PDO and mysqli not available, how can you secure your application? do you force me to use them? what about other languages other than PHP? I prefer to provide general ideas as it can be used for wider border not just for specific language.

SQL user (limiting user privilege) : most common SQL operations are (SELECT, UPDATE, INSERT), then, why giving UPDATE privilege to a user that not require it? for example: login, and search pages are only using SELECT, then, why using db users in these pages with high privileges? 
RULE: do not create one db user for all privileges, for all SQL operations, you can create your scheme like (deluser, selectuser, updateuser) as usernames for easy usage.
see Principle of least privilege

Data filtering: before building any query user input should be validated and filtered, for programmers, its important to define some properties for each user-input variables:
data type, data pattern, and data length. a field that is a number between (x and y) must be exactly validated using exact rule, for a field that is a string (text): pattern is the case, for example: username must contain only some characters lets say [a-zA-Z0-9_-.] the length vary between (x and n) where x and n (integers, x <=n ).
Rule: creating exact filters and validation rules are best practice for me. 
Use other tools: Here, I will also agree with you that prepared statement (parametrized query) and Stored procedures, the disadvantages here is these ways requires advanced skills which are not exist in most users, the basic idea here is to distinguish between SQL query and the data that being used inside, both approach can be used even with unsafe data, because the user-input data here not add anything to the original query such as (any or x=x).
for more information please read OWASP SQL Injection Prevention Cheat Sheet.
Now, if you are an advanced user, start using these defense as you like, but, for beginners, if they can't quickly implement stored procedure and prepared statement, its better to filter input data as much they can.

Finally, lets consider that user sends this text below instead of entering his  username:

[1] UNION SELECT IF(SUBSTRING(Password,1,1)='2',BENCHMARK(100000,SHA1(1)),0) User,Password FROM mysql.user WHERE User = 'root'


This input can be checked early without any prepared statement and stored procedures, but to be on safe side, using them starts after user-data filtering and validation.

Last point is detecting unexpected behavior which requires more effort and complexity, its not recommended for normal web applications.
Unexpected behavior in above user input is: SELECT, UNION, IF, SUBSTRING, BENCHMARK, SHA, root once these words detected, you can avoid the input.

UPDATE1:

User commented that this post is useless, ok! here is what OWASP.ORG provided:


  Primary Defenses:

Option #1: Use of Prepared Statements (Parameterized Queries)
Option #2: Use of Stored Procedures
Option #3: Escaping all User Supplied Input 

  
  Additional Defenses:

Also Enforce: Least Privilege
Also Perform: White List Input Validation



As you may knew, claiming on any article should be supported by valid argument, at least one reference! otherwise its considered as attack and bad claim!

Update2:

From PHP Manual http://php.net/manual/en/mysqli.quickstart.prepared-statements.php:


  Escaping and SQL injection
  
  Bound variables will be escaped automatically by the server. The
  server inserts their escaped values at the appropriate places into the
  statement template before execution. A hint must be provided to the
  server for the type of bound variable, to create an appropriate
  conversion. See the mysqli_stmt_bind_param() function for more
  information.
  
  The automatic escaping of values within the server is sometimes
  considered a security feature to prevent SQL injection. The same
  degree of security can be achieved with non-prepared statements, if
  input values are escaped correctly.


Update3:

I created test cases for knowing how PDO and MySQLi sends the query to MySQL server when using prepared statement:

PDO:

$user = "''1''"; //malicious keyword
$sql = 'SELECT * FROM awa_user WHERE userame =:username';
$sth = $dbh->prepare($sql, array(PDO::ATTR_CURSOR => PDO::CURSOR_FWDONLY));
$sth->execute(array(':username' => $user));


Query Log:


    189 Query SELECT * FROM awa_user WHERE userame ='\'\'1\'\''
    189 Quit



MySQLi:

$stmt = $mysqli->prepare("SELECT * FROM awa_user WHERE username =?")) {
$stmt->bind_param("s", $user);
$user = "''1''";
$stmt->execute();


Query Log:


    188 Prepare   SELECT * FROM awa_user WHERE username =?
    188 Execute   SELECT * FROM awa_user WHERE username ='\'\'1\'\''
    188 Quit



Its clear that prepared statement also escaping the data, nothing else.

As also mentioned in above statement The automatic escaping of values within the server is sometimes considered a security feature to prevent SQL injection. The same degree of security can be achieved with non-prepared statements, if input values are escaped correctly, therefore, this proves that data validation such as intval() is a good idea for integer values before sending any query, in addition, preventing malicious user data before sending the query is correct and valid approach.

Please see this question for more detail: PDO sends raw query to MySQL while Mysqli sends prepared query, both produce the same result

References:

SQL Injection Cheat Sheet
SQL Injection
Information security
Security Principles
Data validation
0
A few guidelines for escaping special characters in SQL statements.

Don't use MySQL, this extension is deprecated, use MySQLi or PDO.

MySQLi

For manually escaping special characters in string you can use mysqli_real_escape_string function. The function will not work properly unless the correct character set is set with mysqli_set_charset.

Example:

$mysqli = new mysqli( 'host', 'user', 'password', 'database' );
$mysqli->set_charset( 'charset');

$string = $mysqli->real_escape_string( $string );
$mysqli->query( "INSERT INTO table (column) VALUES ('$string')" );


For automatic escaping of values with prepared statements, use mysqli_prepare, and mysqli_stmt_bind_param where types for the corresponding bind variables must be provided for an appropriate conversion:

Example:

$stmt = $mysqli->prepare( "INSERT INTO table ( column1, column2 ) VALUES (?,?)" );

$stmt->bind_param( "is", $integer, $string );

$stmt->execute();


No matter if you use prepared statements or mysqli_real_escape_string, you always have to know the type of input data you're working with. 

So if you use prepared statement, you must specify the types of the variables for mysqli_stmt_bind_param function.

And use of mysqli_real_escape_string is for, as the name says, escaping special characters in a string, so it will not make integers safe. The purpose of this function is to prevent breaking the strings in sql statements, and the damage to the database that it could cause. mysqli_real_escape_string is a useful function when used properly, especially when combined with sprintf.

Example:

$string = "x' OR name LIKE '%John%";
$integer = '5 OR id != 0';

$query = sprintf( "SELECT id, email, pass, name FROM members WHERE email ='%s' AND id = %d", $mysqli->real_escape_string( $string ), $integer );

echo $query;
// SELECT id, email, pass, name FROM members WHERE email ='x\' OR name LIKE \'%John%' AND id = 5

$integer = '99999999999999999999';
$query = sprintf( "SELECT id, email, pass, name FROM members WHERE email ='%s' AND id = %d", $mysqli->real_escape_string( $string ), $integer );

echo $query;
// SELECT id, email, pass, name FROM members WHERE email ='x\' OR name LIKE \'%John%' AND id = 2147483647 

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How protect index.php by sql-injection using .htaccess was not really a duplicate! Some have reported it as a duplicate question to this question. So, I'll post my answer here, an admin can even move my answer from here to a so called duplicate question:

However, if the attackers is trying to hack with the form via PHP $_GET variable or with the URL's query string, you can able to catch them if they're not secure.

RewriteCond %{QUERY_STRING} ([0-9]+)=([0-9]+)
RewriteRule ^(.*) ^/track.php


Because 1=1, 2=2, 1=2, 2=1, 1+1=2, etc... are the common questions to SQL database of an attacker. Maybe also it's used by many hacking applications. But you must be careful, that you must not rewrite a safe queries from your site. The code above is giving you a tip, to rewrite or redirect (depends on you) that hacking-specific dynamic query string into a page that will store the attacker's I.P. address, or EVEN THEIR COOKIES, history, browser, or any other sensitive informations, and try to hack them back for security purpose.
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A simple way would be to use a PHP framework like CodeIgniter or Laravel which have in-built features like filtering and active-record, so that you dont have to worry about these nuances. 
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I think PDO is a great method, but I also think that there are some great Mapping method that can help prevent SQL injection also. 

For example: the ORM that comes with Kohana.Working Example

Read More Here:
http://kohanaframework.org/3.0/guide/orm. 

I like this method also and have used it in several PHP applications.

Additionally:
You can set a YAML file outside your web source directory on the server to handle the orig DB config options too will help prevent SQL Injection.
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Parameterized query AND input validation is the way to go. There is many scenarios under which SQL injection may occur, even though mysql_real_escape_string() has been used.

Those examples are vulnerable to SQL injection :

$offset = isset($_GET['o']) ? $_GET['o'] : 0;
$offset = mysql_real_escape_string($offset);
RunQuery("SELECT userid, username FROM sql_injection_test LIMIT $offset, 10");


or

$order = isset($_GET['o']) ? $_GET['o'] : 'userid';
$order = mysql_real_escape_string($order);
RunQuery("SELECT userid, username FROM sql_injection_test ORDER BY `$order`");


In both case you can't use ' to protect the encapsulation.

source : The Unexpected SQL Injection (When Escaping Is Not Enough)
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