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);

?>

Documentation

Related : Mutex, Concurrency, Shared Memory, Thread, Green Thread