You should explain that the block size is return in BYTES, not the more commonly used unit, bits. Many readers are not going to know the DES block size and figure this out on their own.
(PHP 4, PHP 5)
mcrypt_get_block_size — Gets the block size of the specified cipher
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).
One of the MCRYPT_ciphername constants or the name of the algorithm as string.
Gets the block size, as an integer.
Example #1 mcrypt_get_block_size() example
This example shows how to use this function when linked against libmcrypt 2.4.x and 2.5.x.
echo mcrypt_get_block_size('tripledes', 'ecb'); // 8