PHP Manual: Decrypts crypttext with given parameters

string mcrypt_decrypt ( string $cipher , string $key , string $data , string $mode [, string $iv ] )

Decrypts the data and returns the unencrypted data.

cipher: One of the MCRYPT_ciphername constants, or the name of the algorithm as string.
key: The key with which the data was encrypted. If it's smaller than the required keysize, it is padded with '\0'.
data: The data that will be decrypted with the given cipher and mode. If the size of the data is not n * blocksize, the data will be padded with '\0'.
mode: One of the MCRYPT_MODE_modename constants, or one of the following strings: "ecb", "cbc", "cfb", "ofb", "nofb" or "stream".
iv: Used for the initialization in CBC, CFB, OFB modes, and in some algorithms in STREAM mode. If you do not supply an IV, while it is needed for an algorithm, the function issues a warning and uses an IV with all its bytes set to "\0".

Returns the decrypted data as a string.