Gets the size of the IV belonging to a specific cipher/mode combination.
It is more useful to use the mcrypt_enc_get_iv_size function as this uses the resource returned by mcrypt_module_open.
One of the MCRYPT_ciphername constants, or the name of the algorithm as string.
One of the MCRYPT_MODE_modename constants, or one of the following strings: "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.
Returns the size of the Initialization Vector (IV) in bytes. On error the function returns false. If the IV is ignored in the specified cipher/mode combination zero is returned.