In a comment Carriantor has pointed me to a tutorial (which has disappeared in the meantime, please try the tutorial from José Pablo Barrantes) about integrating phpDocumentor, a tool to generate API docs, into Eclipse. Thanks Carriantor!
Update 2008-06-21: Changed the link for the tutorial as the tutorial no longer exists at the original url.
Update 2008-07-28: Adding link to Simon Speich’s tutorial.
Update 2010-04-29: Fixing/removing broken links
Update 2010-05-10: Adding link to the English version of Simon Speich’s tutorial