PHP: mysql_real_escape_string

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mysql_real_escape_string

Description

string mysql_real_escape_string ( string unescaped_string [, resource link_identifier] )

Escapes 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_real_escape_string, which prepends backslashes to the following characters: \x00, \n, \r, \, ', " and \x1a.

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

Parameters

unescaped_string: The string that is to be escaped.
link_identifier: The MySQL connection. If the link identifier is not specified, the last link opened by mysql_connect() is assumed. If no such link is found, it will try to create one as if mysql_connect() was called with no arguments. If by chance no connection is found or established, an E_WARNING level warning is generated.

Return Values

Returns the escaped string, or FALSE on error.

Examples

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));
?>

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 user='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

if (isset($_POST['product_name']) && isset($_POST['product_description']) && isset($_POST['user_id'])) {
    // Connect

    $link = mysql_connect('mysql_host', 'mysql_user', 'mysql_password');

    if(!is_resource($link)) {

        echo "Failed to connect to the server\n";
        // ... log the error properly

    } else {
        
        // Reverse magic_quotes_gpc effects on those vars if ON.

        if(get_magic_quotes_gpc()) {
            $product_name        = stripslashes($_POST['product_name']);
            $product_description = stripslashes($_POST['product_description']);
        } else {
            $product_name        = $_POST['product_name'];
            $product_description = $_POST['product_description'];
        }

        // Make a safe query
        $query = sprintf("INSERT INTO products (`name`, `description`, `user_id`) VALUES ('%s', '%s', '%d')",
                    mysql_real_escape_string($product_name, $link),
                    mysql_real_escape_string($product_description, $link),
                    $_POST['user_id']);

        mysql_query($query, $link);

        if (mysql_affected_rows($link) > 0) {
            echo "Product inserted\n";
        }
    }
} else {
    echo "Fill the form properly\n";
}
?>

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

Notes

Note: A MySQL connection is required before using mysql_real_escape_string() otherwise an error of level E_WARNING is generated, and FALSE is returned. If link_identifier isn't defined, the last MySQL connection is used.

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.

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

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

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