The first prototype is when linked against libmcrypt 2.2.x, the second when linked against libmcrypt 2.4.x.
mcrypt_cfb() encrypts or decrypts (depending on mode) the data with cipher and key in CFB cipher mode and returns the resulting string.
Cipher is one of the MCRYPT_ciphername constants.
Key is the key supplied to the algorithm. It must be kept secret.
Data is the data which shall be encrypted/decrypted.
Mode is MCRYPT_ENCRYPT or MCRYPT_DECRYPT.
IV is the initialization vector.