A Comet implementation in Php. The first callback should be some function that's waiting for an external result, which it will return (otherwise, it should not return anything), the second callback is the function to invoke with the result of the first callback. Thats it.

CakePHP Comet Lib

<?php
define('MESSAGE_POLL_MICROSECONDS', 500000);
define('MESSAGE_TIMEOUT_SECONDS', 60);
define('MESSAGE_TIMEOUT_SECONDS_BUFFER', 5);
 
class Comet {
  public $work;
	public $callback;

	public static function &init($work, $callback) {
		session_write_close();
		set_time_limit(MESSAGE_TIMEOUT_SECONDS+MESSAGE_TIMEOUT_SECONDS_BUFFER);
		$instance = new self($work,$callback);
		return $instance;
	}
	
	public function __construct($work,$callback) {
		$this->work = $work;
		$this->callback = $callback;
	}	

	public function poll() {
		$counter = MESSAGE_TIMEOUT_SECONDS;
		$data = null;
		while($counter > 0) {
				if ($data = call_user_func($this->work)) {
						break;
				} else {
						usleep(MESSAGE_POLL_MICROSECONDS);
						$counter -= MESSAGE_POLL_MICROSECONDS / 1000000;
				}
		}
		call_user_func_array($this->callback, array($data));
	}
}