ECLIPSE -> PHP Development Tools Project "The PDT project provides a PHP Development Tools framework for the Eclipse platform. This project encompasses all development components necessary to develop PHP and facilitate extensibility. It leverages the existing Web Tools Platform (WTP) and Dynamic Languages Toolkit (DLTK) in providing developers with PHP capabilities." I would suggest Eclipse if you are working for an educational project or a large web PHP application project. vim/gvim would be a choice if you have only a few php files in your website..
IMO, NetBeans and Notepad++ ( not an IDE but does the job very well if I need to change a few lines here and there ).