I couldn't get this working for a while. I had a stock eclipse setup from the Ubuntu repository that I had put Flex Builder in. Next went to install PDT but couldn't get it working whether through a repository, through the eclipse update site, etc.

I didn't realize you need to run eclipse as root to install plugins. and it provides no error message telling you the installation was a complete failure- it just fails to write the plugin files silently. doh!

Here's how to get it working:

- start eclipse as root - sudo eclipse
- Help > Install new software
- Use this update site http://download.eclipse.org/releases/galileo/
- Search for "php" and install that package
- restart

Thanks to this: http://ubuntuforums.org/showthread.php?p=9231710#post9231710