In this most recent post to PHPMaster.com Alex Stetsenko takes a look at the Zend Job Queue functionality, a part of the Zend Server installation. He talks about some basic usage to make HTTP requests and a more extended example showing report generation.

Web applications usually follow a synchronous communication model. However, non-interactive and long-running tasks (such as report generation) are better suited for asynchronous execution. One way to off-load tasks to run at a later time, or even on a different server, is use the Job Queue module available as a part of Zend Server 5 (though not as part of the Community Edition). Job Queue allows job scheduling based on time, priority, and even dependencies

In his two examples, he shows the code involved to create a new Queue object and define a HttpJob in it. The first just calls a "sample.php" script that's exposed as a part of your external-facing site and shows how you can get the current status of the job. The more advanced example shows a call to a "report.php" script with a set of options defining things like "type", "length" and "priority". He also points out some other options that can do similar things like Gearman, NodeJs and RabbitMQ.