Situation: I upgraded a debian distribution wheezy to jessie with no significant issues. After upgrading I experienced that some of the already installed and formerly working php scripts stopped working. Content of some included php scripts, e.g. wordpress configuration, were printed as HTML content instead of being evaluated.
Reason: The updated php.ini disables short open tags by default. Check your affected php files whether they use the long php open tags [code]<?php[/code] instead of (my preferred) short open tags [code]<?[/code].
Solution: Either you replace all short php open tags by long php open tags or you can enable the use of short php open tags system-wide. The latter was more comfortable in my case, as I do not know how many files would be affected. To do so, check the installed php.ini used by your apache installation (typically located at /etc/apache2/php.ini) for [code]short_open_tag = On[/code] and restart your apache server [code]apache2ctrl restart[/code].