In a comment Carriantor has pointed me to a tutorial about integrating phpDocumentor, a tool to generate API docs, into Eclipse. Thanks Carriantor!
Another short tutorial about this topic has been written by Simon Speich.
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.
Well thats nice but too bad that the tutorial lin goes to a 404 not found page.
@Webconsult: Thanks for the hint, I changed the link so that it points now to archive.org where the tutorial is still available.
If you would like you can link to this tutorial instead of the archived one http://www.speich.net/computer/artikel/integration-phpdocumentor-eclipse-en.php
@Simon: Thanks for the link! I added it to the article.