The first prototype is when linked against libmcrypt 2.2.x, the
second when linked against libmcrypt 2.4.x or 2.5.x.
mcrypt_get_key_size() is used to get the size
of a key of the specified cipher
(in
combination with an encryption mode).
This example shows how to use this function when linked against libmcrypt
2.4.x and 2.5.x. It is more useful to use the
mcrypt_enc_get_key_size() function as this uses the
resource returned by mcrypt_module_open().