PHP mysql_real_escape_string function

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mysql_real_escape_string

(PHP 4 >= 4.3.0, PHP 5)

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.

The string that is to be escaped.

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.

Returns the escaped string, or FALSE on error.

Example 1. Simple mysql_real_escape_string() example

// 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

// 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:

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.

 
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.

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.