Very useful when making daemons, for example:

<?php
    $pid = pcntl_fork(); // fork
    if ($pid < 0)
        exit;
    else if ($pid) // parent
        exit;
    else { // child
    
        $sid = posix_setsid();
        
        if ($sid < 0)
            exit;
            
        for($i = 0; $i <= 60; $i++) { // do something for 5 minutes
            sleep(5);
        }
    }
?>