fork_test.php

<?php
class Test {
	private $_num;
	private $_max_process_num = 5;

	public function __construct($num) {
		$this->_num = $num;
	}

	public function run() {
		$pids = array();
		for ($i = 0; $i < $this->_num; $i++) {
			$pid = pcntl_fork();
			var_dump($pid);
			if ($pid == -1) {
				throw new Exception('failed');
			} else if ($pid) {
				// parent
				$pids[$pid] = true;
				if (count($pids) > $this->_max_process_num) {
					$wait_pid = pcntl_waitpid(-1, $status, WUNTRACED);
					unset($pids[$wait_pid]);
				}
			} else {
				// child

				// working for child
				var_dump(array($i, $pid));
				exit;
			}
		}
		while (count($pids) > 0) {
			$wait_pid = pcntl_waitpid(-1, $status, WUNTRACED);
			unset($pids[$wait_pid]);
		}
	}
}

$t = new Test(5);
$t->run();