mcrypt_decrypt() decrypts the data and returns the unencrypted data.
Cipher is one of the MCRYPT_ciphername constants of the name of the algorithm as string.
Key is the key with which the data is encrypted. If it's smaller that the required keysize, it is padded with '\0'.
Data is 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 is one of the MCRYPT_MODE_modename constants of one of "ecb", "cbc", "cfb", "ofb", "nofb" or "stream".
The IV parameter is used for the initialisation 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 bytes set to '\0'.