Check backwards compatibility

[Kizman]

Hi,

This is the situation:

I've developed a PHP application based on version 5.

Now I need to change the web hoster because of a missing functionality in the hosting package.
I've found one which fits my requirements almost perfectly. Unfortunately their version of PHP is 4.3.x.

I would need to adapt my PHP script so that it is compatible with lower version, but this is a huge work!
Actually I am wondering if there is a tool which would allow me to check the backwards compatibility of my script, and would pinpoint the differences to adapt.

Does something like that exist?

Thanx for your help!