WARNING: this function WILL NOT destroy the memory segment if there are other processes attached to it, but it will return TRUE anyways!

You will have to try to access the segment again after destroying it to make sure it was relly destroyed.

It appears that you must be connected to the shm segment in order to remove it.

<?php
shm_remove($shm);
shm_detach($shm);
?>

works, while

<?php
shm_detach($shm);
shm_remove($shm);
?>

produces "Warning: shm_remove(): The parameter is not a valid shm_identifier ..."

Though it says **shm_identifier** in
  int shm_remove(int shm_identifier);
and
  shm_attach() returns an **id** that that can be used to access the System V...

a snippet like this:
<?php
  $key = 1234;
  $shm=shm_attach($key);
  // do whatever...
  shm_remove($shm);
?>

produces an error like
Warning: 2(which is the value of $shm) is not a existing SysV shared memory key in /usr/cu/web/sema.php on line 44...

The documentation confused me a bit. I thought shm_remove would get the shm handle/id that returns from shm_attach, not the key given to shm_attach. So, use instead:
  shm_remove($key);

which is what is intended indeed.