[Thunderbird8]

Using PHP 5.5 will cause a few deprecated errors to show up though, as one of the changes was to make the /e modifier for preg_replace deprecated. vB uses this in a few places. I believe most of them were fixed in 4.2.2, but I've caught a couple instances still occurring in my error logs (though 4.2.2 vastly reduced the error spew from vB to the logs on 5.4 and 5.5).

If you use vBSEO, you'll get even more errors from running PHP 5.5 (I've fixed at least 6 different locations by now in my own installation from that, but there's one in the configuration area that I've not been able to fix without having the configuration get blanked out).

TL;DR version, stick with PHP 5.4 for now, unless you have a need for 5.5.