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mysql_real_escape_string

(PHP 4 >= 4.3.0, PHP 5)

mysql_real_escape_string --  Escapes special characters in a string for use in a SQL statement

Description

string mysql_real_escape_string ( string unescaped_string [, resource link_identifier])

unescaped_string

The string to escape

link_identifier (optional)

The mysql connection resource

This function will escape special characters in the unescaped_string, taking into account the current character set of the connection so that it is safe to place it in a mysql_query(). If binary data is to be inserted, this function must be used.

mysql_real_escape_string() calls MySQL's library function mysql_escape_string, which prepends backslashes to the following characters: NULL, \x00, \n, \r, \, ', " and \x1a.

Example 1. Simple mysql_real_escape_string() example

<?php
// Connect
$link = mysql_connect('mysql_host', 'mysql_user', 'mysql_password')
    OR die(mysql_error());

// Query
$query = sprintf("SELECT * FROM users WHERE user='%s' AND password='%s'",
            mysql_real_escape_string($user),
            mysql_real_escape_string($password));
?>

This function must always (with few exceptions) be used to make data safe before sending a query to MySQL.

Note: If magic_quotes_gpc is enabled, first apply stripslashes() to the data. Using this function on data which has already been escaped will escape the data twice.

If this function is not used to escape data, the query is vulnerable to SQL Injection Attacks.

Example 2. An example SQL Injection Attack

<?php
// Query database to check if there are any matching users
$query = "SELECT * FROM users WHERE user='{$_POST['username']}' AND password='{$_POST['password']}'";
mysql_query($query);

// We didn't check $_POST['password'], it could be anything the user wanted! For example:
$_POST['username'] = 'aidan';
$_POST['password'] = "' OR ''='";

// This means the query sent to MySQL would be:
echo $query;
?>

The query sent to MySQL: 

SELECT * FROM users WHERE name='aidan' AND password='' OR ''=''

This would allow anyone to log in without a valid password.

Example 3. A "Best Practice" query

Using mysql_real_escape_string() around each variable prevents SQL Injection. This example demonstrates the "best practice" method for querying a database, independent of the Magic Quotes setting.

<?php
// Quote variable to make safe
function quote_smart($value)
{
    // Stripslashes
    if (get_magic_quotes_gpc()) {
        $value = stripslashes($value);
    }
    // Quote if not integer
    if (!is_numeric($value)) {
        $value = "'" . mysql_real_escape_string($value) . "'";
    }
    return $value;
}

// Connect
$link = mysql_connect('mysql_host', 'mysql_user', 'mysql_password')
    OR die(mysql_error());

// Make a safe query
$query = sprintf("SELECT * FROM users WHERE user=%s AND password=%s",
            quote_smart($_POST['username']),
            quote_smart($_POST['password']));

mysql_query($query);
?>

The query will now execute correctly, and SQL Injection attacks will not work.

Note: mysql_real_escape_string() does not escape % and _. These are wildcards in MySQL if combined with LIKE, GRANT, or REVOKE.

See also mysql_client_encoding(), addslashes(), stripslashes(), the magic_quotes_gpc, and the magic_quotes_runtime directive.

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