Question

How to make functions in PHP synchronized so that same function won’t be executed concurrently ? 2nd user must wait till 1st user is done with the function. Then 2nd user can execute the function.

Thanks

Answer 1

This basically comes down to setting a flag somewhere that the function is locked and cannot be executed until the first caller returns from that function.
This can be done in a number of ways:

• use a lock file (first function locks a file name “f.lok”, second function checks if the lock file exists and executes or doesn’t based on that evaluation)
• set a flag in the database (not recomended)
• use semaphores as @JvdBerg suggested (the fastest)

When coding concurrent application always beware of race conditions and deadlocks!

UPDATE
using semaphores (not tested):

<?php

define('SEM_KEY', 1000);

function noconcurrency() {
    $semRes = sem_get(SEM_KEY, 1, 0666, 0); // get the resource for the semaphore

    if(sem_acquire($semRes)) { // try to acquire the semaphore. this function will block until the sem will be available
        // do the work 
        sem_release($semRes); // release the semaphore so other process can use it
    }
}

PHP needs to be compiled with sysvsem support in order to use sem_* functions

Here’s a more in depth tutorial for using semaphores in PHP:

http://www.re-cycledair.com/php-dark-arts-semaphores