PHP Cookbook

5.6. Sharing Variables Between Processes

Problem

You want a way to share information between processes that provides fast access to the shared data.

Solution

Store the data in a shared memory segment, and guarantee exclusive access to the shared memory with a semaphore:

$semaphore_id = 100;
$segment_id   = 200;
// get a handle to the semaphore associated with the shared memory
// segment we want
$sem = sem_get($semaphore_id,1,0600);
// ensure exclusive access to the semaphore
sem_acquire($sem) or die("Can't acquire semaphore");
// get a handle to our shared memory segment
$shm = shm_attach($segment_id,16384,0600);
// retrieve a value from the shared memory segment
$population = shm_get_var($shm,'population');
// manipulate the value
$population += ($births + $immigrants - $deaths - $emigrants);
// store the value back in the shared memory segment
shm_put_var($shm,'population',$population);
// release the handle to the shared memory segment
shm_detach($shm);
// release the semaphore so other processes can acquire it
sem_release($sem);

Discussion

A shared memory segment is a slice of your machine’s RAM that different processes (such as the multiple web server processes that handle requests) can access. A semaphore makes sure that the different processes don’t step on each other’s toes when they access the shared memory segment. Before a process can use the segment, it needs to get control of the semaphore. When it’s done with the segment, it releases the semaphore for another process ...