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.
See the article here:
PHPMaster: Zend Job Queue