Try to acquire a specific lock, and then exit.
function system_test_lock_exit() {
if (lock()
->acquire('system_test_lock_exit', 900)) {
echo 'TRUE: Lock successfully acquired in system_test_lock_exit()';
// The shut-down function should release the lock.
exit;
}
else {
return 'FALSE: Lock not acquired in system_test_lock_exit()';
}
}