I discovered the Eclipse IDE this year while working with Java, and found to be the nicest example of an IDE I ever used. Now happily it is now available for PHP. Very nice...

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Updated 2021 : note that the above post is out-of-date, given this post was originally published in 2004, but is left here for archival purposes. The old link http://www.phpeclipse.de/tiki-view_articles.php is no longer valid, so I have unlinked it.