I just tried PHP 5.0.0 on my test server and it mostly worked with Horde 2.2.5 and IMP 3.2.4. The only problem I have run into so far is in the compose window. I get the following error when I click "Send" Fatal error: Using $this when not in object context in /usr/local/apache-test/php/lib/php/Mail/RFC822.php on line 168