Run external program in background

[eluser]esra[/eluser]
A PECL module for PHP threading exists, but the module is a several years old, is buggy and no longer appears to be maintained.

There is a class on phpclasses.org that allows an application to spawn another HTTP session for background processing. There is no inter-application communication, but the first application can detect when the process is completed by checking if the new file exists. This has been used a lot pdf processing, xslt transformations, etc.