Right now I'd say the only major "jump" that exists is the one between PHP 4 (released +10 years ago) and PHP 5. Other than that, apps are largely backwards compatible between 5.x versions, except for a few minor changes that no dev really stresses about.

The thing about PHP is that since it's massively used around the net, rather than forcing devs to catch up with the language, PHP devs themselves try as much as possible not to break backwards compability. Which in many ways is a pity, but that seems to be the direction of the language at the moment.