Errore classe mcrypt

[lucidafollia]

così ad occhio

<?php
class Esempio{
           public $algo = 'sha512';
           public $cipher = MCRYPT_RIJNDAEL_128;
           public $mode = MCRYPT_MODE_CBC;
           public $iv = MCRYPT_RAND;
           public $ot = 0;

public function __construct($str, $class){
             $this->$mcrypt = $class;
             $this->iv_size = mcrypt_get_iv_size($this->mcrypt->cipher, $this->mcrypt->mode);
             $this->key = $this->key($str);
 }

}
 
$mcrypt = new mcrypt('!!salt!!', new Esempio);
$encrypt = $mcrypt->encrypt('sak32009');
$decrypt = $mcrypt->decrypt($encrypt);
echo "$decrypt";
?>

In questo modo l'erroe è diventato:
Fatal error: Class 'mcrypt' not found