shmop_size() is used to get the
size, in bytes of the shared memory block.
shmop_size() takes the shmid,
which is the shared memory block identifier created by
shmop_open(), the function will return and int, which
represents the number of bytes the shared memory block
occupies.
This example will put the size of shared memory block
identified by $shm_id into $shm_size.