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_block_size() is used to get the size of a block 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.
See also: mcrypt_get_key_size() and mcrypt_encrypt().