This a simple multi process application where you can choose 
the maximun process that can run at the same time.
This is useful when you need to limit the fork of process.
When the MAXPROCESS is reached the program wait on pcntl_wait()
<?php
DEFINE(MAXPROCESS,25);
for ($i=0;$i<100;$i++){
 $pid = pcntl_fork();
 
 if ($pid == -1) {
 die("could not fork");
 } elseif ($pid) {
 echo "I'm the Parent $i\n";
 $execute++;
 if ($execute>=MAXPROCESS){
 pcntl_wait($status);
 $execute--;
 }
 } else {
 echo "I am the child, $i pid = $pid \n";
 sleep(rand(1,3));
 echo "Bye Bye from $i\n"; 
 exit;
 }
}
?>