If you have done PHP development work in other environments, you should know about the following PHP environment gotchas on Pubweb.
Short open tags <?
Short open tags (<? as opposed to <?php) are not enabled on PubWeb.
This means you may need to modify old code or code you have borrowed or downloaded from other sources. When <? is used, the page is treated like XML and you will see the PHP code when visiting the page from a web browser. Change these to <?php to make your pages parseable as PHP.
Be sure to search and replace through all of your code, not only the first instance in each file.