Retardo tras autenticación fallida en PHP

 <?php
/* mysql_connect() */
/* mysql_select_db() */
$clean = array();
$mysql = array();

$now = time();
$max = $now - 15;

$salt = 'SHIFLETT';

if (ctype_alnum($_POST['username']))
  {
    $clean['username'] = $_POST['username'];
  }
else
  {
    /* ... */
  }

$clean['password'] = md5($salt . md5($_POST['password'] . $salt));
$mysql['username'] = mysql_real_escape_string($clean['username']);
$sql = "SELECT last_failure, password
     FROM users
     WHERE username = '{$mysql['username']}'";

if ($result = mysql_query($sql))
  {
    if (mysql_num_rows($result))
      {
        $record = mysql_fetch_assoc($result);
          if ($record['last_failure']> $max)
            {
              /* Less than 15 seconds since last failure */
            }
          elseif ($record['password'] == $clean['password'])
            {
              /* Successful Login */
            }
          else
            {
              /* Failed Login */
              $sql = "UPDATE users
                   SET last_failure = '$now'
                   WHERE username = '{$mysql['username']}'";
              mysql_query($sql);
            }
       }
     else
       {
          /* Invalid Username */
       }
     }
else
{
   /* Error */
}
?>