int mcrypt_cbc(int cipher, string key, string data, int mode, string [iv]);
mcrypt_cbc() encrypts or decrypts (depending on mode) the data with cipher and key in CBC 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 optional initialization vector.