mcrypt_get_iv_size() returns the size of
the Initialisation Vector (IV) in bytes. On error the function
FALSE. If the IV is ignored in the specified cipher/mode
combination zero is returned.
cipher is one of the MCRYPT_ciphername constants of the name of the algorithm as string.
mode is one of the MCRYPT_MODE_modename constants or one of "ecb", "cbc", "cfb", "ofb", "nofb" or "stream". The IV is ignored in ECB mode as this mode does not require it. You will need to have the same IV (think: starting point) both at encryption and decryption stages, otherwise your encryption will fail.
echo mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB) . "\n";
echo mcrypt_get_iv_size('des', 'ecb') . "\n";