Optimizing mcrypt encryption class further
You can actually refactor most of the code into a separate function:

static function encryptdecrypt($text)
{
    $td = mcrypt_module_open(self::CIPHER, "", self::MODE, "");
    mcrypt_generic_init($td, self::KEY, 0);
    $text = mcrypt_generic($td, $text);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    return $text;
}

static function encrypt($plaintext)
{
    return base64_encode(encryptdecrypt($plaintext));
}

static function decrypt($ciphertext)
{
    return encryptdecrypt(base64_decode($ciphertext));
}