The size of the initialization vector (iv) will be returned in bytes NOT bits, something to keep in mind specially if the iv is passed together appended or prepended with the encrypted data for decryption
(PHP 4 >= 4.0.2, PHP 5)
mcrypt_get_iv_size — Returns the size of the IV belonging to a specific cipher/mode combination
Gets the size of the IV belonging to a specific cipher/mode combination.
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.
Example #1 mcrypt_get_iv_size() Example
echo mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB) . "\n";
echo mcrypt_get_iv_size('des', 'ecb') . "\n";