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'.