Problem 1: The old versions do not get security patches. For example, entering the number 2.2250738585072011e-308 hangs the interpreter in old versions, as far as I know this is unpatched in PHP <=5.1. This allows a very effective denial of service.

Problem 2: You are delaying the unevitable; it's nice to use new features of the language, having to code in old versions is a pain for developers. Small continuous upgrades are easier to handle than rare gigantic ones.