How to make PHP 5.2 compatible with future versions

I am writing a CMS in version 5.2 but I am worried whether or not it will be compatible with PHP 5.3 or future versions for that matter.

What things I should watch out for or there is nothing to worry about ?

P.S: Please don't suggest that I should use PHP 5.3 instead as that's not the question asked.

Solution

PHP takes a very conservative approach to backwards compatibility. Most code will continue to work for quite a while - I've seen PHP3 code that's still OK in 5.3.

You should keep an eye on the lists of backwards incompatible changes.