A Licensing Question

I was wondering what the licensing issues for app frameworks (or any included PHP classes/libs) are. I'm creating a rather large PHP application for a company, and they'd like to potentially resell it to some of their customers. Can I safely use GPL'd libraries, classes, etc. (i.e. PearDB) and resources that fall under other open source licenses when developing the app and let them redistribute accordingly, or would that be a violation? Also, thinking ahead here a bit, if they wanted to use something like Zend Encoder on the project, what sort of added gotchas could that bring up? Thanks guys! John