Zend provides a preview version of the upcoming PHP IDE which I mentioned in another blog entry. They provide a short introduction how to install this Eclipse extension.
Update: Thanks to Martin who pointed out some installation problems (see comment), Linux users should care for the following:
A package named „WST“ is needed (at least for the 3.1 linux version) which is available from http://download.eclipse.org/webtools/updates/
via the same update mechanism.