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
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'.