It's usually good to know some older versions of the program and not just the latest version.
it really depends on what you want to do. If you plan on becoming professional php developer then chances are you will see programs written
in older versions of php, like 5.2 for example.
It's generally good to know since what version a certain feature of php became available.
You should always know that latest stable version of the program and maybe some features from development version, so regardless of whether or not you plan on learning 5.2 you definitely must learn 5.3 and then some of the newest features in 5.4