What about running php script of previous versions in new release?

I'm reading php manual
The syntax changes in php versions are wonderful! I haven't seen such changes in any other languages before.
Some important changes are done without support of older versions. Here is an example:
In php 4 instantiating objects gives a copy of them, but in php 5 it returns a reference to instantiated object.
what should I do with my older scripts to run in a new php engine? Are there some translators or something like that? Am I right that there are big differences?


Thank you all.