------------item------------
<?php
require_once('database.php');
$id = $_GET['serial'];

$data = Database::getItem($id);
if($data != null){
  echo $data['item_id'] . '<br />';
  echo $data['item_name'] . '<br />';
  echo $data['item_serial'] . '<br />';
  echo $data['item_number'] . '<br />';
}
?>

------------index------------
<form action="item.php">
Serial Number:<br>
<input type="text" name="serial" value="">
<br>
<input type="submit">
</form>

------------database------------
<?php
class Database
{
    private static $dbName = 'limiti';
    private static $dbHost = 'localhost';
    private static $dbUsername = 'root';
    private static $dbUserPassword = '';

    private static $cont  = null;

    public function __construct() {
        die('Init function is not allowed');
    }

    public static function connect()
    {
       // One connection through whole application
       if ( null == self::$cont )
       {
        try
        {
          self::$cont =  new PDO( "mysql:host=".self::$dbHost.";"."dbname=".self::$dbName, self::$dbUsername, self::$dbUserPassword);
        }
        catch(PDOException $e)
        {
          die($e->getMessage());
        }
       }
       return self::$cont;
    }

    public static function disconnect()
    {
        self::$cont = null;
    }

    public static function getItem($item_serial)
    {
      $pdo = Database::connect();
      $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
      $sql = "SELECT * FROM  limited WHERE  item_serial = ?";
      $q->execute(array($id));
      $data = $q->fetch(PDO::FETCH_ASSOC);
      Database::disconnect();
      //var_dump($data);
      return $data;
    }


}
?>

------------error------------
Notice: Undefined variable: q in C:\xampp\htdocs\database.php on line 42

Fatal error: Call to a member function execute() on null in C:\xampp\htdocs\database.php on line 42