I recently upgraded my PHP version from 5.2.6 to PHP 5.3.1 and when restarting Apache half my sites suddenly had parse errors. I could not work out what the issue was. Rolling back did not fix the problem, compiling it various times took long but did not solve the problem. What was even more strange is some sites still worked and others didn’t. After a while we finally worked out that ‘short-tags’ in the php.ini file were disabled and some lazy coder who wrote a plug-in that I am using used short tags effectively breaking half my sites. So if you upgrade PHP and WordPress stops working, don’t always assume that the problem is with the upgrade, think of other issues too, in this case short-tags.