What causes this error? Fatal error: Call to undefined function mcrypt_encrypt()
<p>Does anyone know why this error message: <code>(Call to undefined function mcrypt_encrypt() )</code> displays when I run the following code below?</p>

<p>Am I missing some steps perhaps any setting in PHP I have to do before this code can work?</p>

<pre><code>$key = 'password to (en/de)crypt';
$string = 'string to be encrypted';

$test = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, md5($key), $string, MCRYPT_MODE_CBC, md5(md5($key)));
</code></pre>
