What should I focus on as I port a PHP 4 application to the current version of PHP?
<p>I have a couple of old applications which were written in around 2003 in PHP4 which I would like to port to a modern, current version of PHP. I want to first get the old functionality working on a modern server before I start rewriting the code to add feature, modernize the UI and update the code for the current state of the web. </p>

<p>But before I start porting I'd like some guidance about what I should be watching for which has changed - what parts of the code will likely require the most care to rewrite?</p>
