To install latest stable PDT build, open "Help -> Install New Software" in Eclipse and enter following URL:
I found the above solution not to be the case. Instead of working nicely, it comes back with dependency errors, usually that it's missing the PDT UI Plug-in. Searching around their forums gives no help. Either people figured it out and didn't share their solutions with the world (a huge peeve of mine) or they went with the default answer, which is to install the pre-packaged version of Eclipse 3.7/PDT.
Well, I didn't want any of their non-sense, so I searched around for a better way. It turns out that Indigo (the last Eclipse version) has a full release list at /releases/indigo/, so I decided to try it out. It takes some time to load, but not surprisingly, there's a PHP Developer Toolkit listed under the "Programming Languages" section. I picked it out, hit next, and no dependency issues came up. What magic! So, if you're having problems installing PDT as a plugin to Eclipse Classic, use the Juno repository below. It has a million tons of other tools too for your enlightenment. And you know what's in the installed plugins list after this is all done? PDT UI Plug-in.
Juno Repository: http://download.eclipse.org/releases/jun
I hope you find this little tidbit of information a relief to your headaches.