I use the Eclipse devlopment environment withPHPeclipse for my PHP / Drupal coding. It is not a lightweight program but it is very powerful and works fine on my computer with 1 GIG of RAM. I like that it connects to CVS and Subversion ... I have my own Subversion site to keep my code "safe" and many open source projects use CVS so right from the IDE you can get the code and create a new project.

There is quite a learning curve but its not too hard. I don't have the internal debugging working yet but ...

Sun's home page for JAVA is by definition the home page for the language... it was development and mostly still controlled by Sun.