Semaphore
A semaphore is a synchronization primitive used to control access to shared resources in concurrent systems.
Semaphores can be implemented using the sysvsem extension. They are useful for limiting access to a finite number of resources.
<?php
// Create a semaphore
$sem = sem_acquire(sem_get(ftok(__FILE__, 't')));
// Critical section
// ... access shared resource ...
// Release the semaphore
sem_release($sem);
?>