I'm starting to learn PHP. I would like to use PHP 5.3 because it's the newest version so far. I'm working on a project in which I'll use PHP 5.2 scripts that someone gave me.
Are they going to run just fine even though I'm using PHP 5.3?
PHP.net features a guide for upgrading from PHP 5.2.x to PHP 5.3. This includes a section on backwards compatibility.
From my experience, the transition from 5.2 to 5.3 went pretty smoothly smoothly. The only problems I had with my app was to ensure my DateTime setting were property configured in my php.ini, and filter out some overly-agressive deprecation warnings that started showing up.