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.
Small continuous upgrades without ever changing your code is the same cost as a gigantic one without ever changing your code - they either work, or they don't, in variously subtle or spectacular ways. The tipping point lies at some changeset, you just need to hit it.
You upgrade, you may need to change things. It's just a fact of life. Or, you pick a library / language / framework / everything that guarantees 100% backwards compatibility as documented, that never has bugs (since fixing those breaks 100% backwards compatibility), and you never use features in even remotely-unexpected ways. Like in this case.
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.