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A quick post from janschneider.de talks about questions regarding the PHP5 compatibility of Horde, IMP, or other applications on the Horde mailing list
There have been a few questions regarding the PHP5 compatibility of Horde, IMP, or other applications on the Horde mailing list. Even more people have reported problems with Horde on PHP5 powered servers without even considering that they might be incompatible. Additionally I see some search engine hits on Horde and PHP5 on my website, so here is the complete story.
The quick answer, Horde 2.x, IMP 3.x, and all compatible versions do not work with PHP5 and never will. Horde 3, IMP 4, etc. will be compatible and the current development code of these upcoming versions already seems to work fine with PHP5.
Bad news for all of the older Horde users, but, thankfully, there will be an upgrade soon that will allow you to make the switch.