Set the real group ID of the current process. This is a privileged function and needs appropriate privileges (usually root) on the system to be able to perform this function. The appropriate order of function calls is posix_setgid() first, posix_setuid() last.
If the caller is a super user, this will also set the effective group id.
This example will print out the effective group id, once it is changed.
echo 'My real group id is '.posix_getgid(); //20
echo 'My real group id is '.posix_getgid(); //40
echo 'My effective group id is '.posix_getegid(); //40