[xorcist]

> PHP [...] at least allows you to run 4 code on the 5 interpreter with no issues

Speaking as someone who at one time made a living porting PHP4 code to PHP5, that's not even the same ballpark as reality. No non-trivial code base made that transition without effort. But it is of course possible to write code that runs on both versions, which may be the source of the confusion.

Guess why all cheap web hosting companies only offer ancient versions of PHP? "I downloaded $crap from teh Internet and it doesn't work" is nobody's favourite idea of customer support.

If I were to think of a scripting language (to exclude C) that has kept backwards compatibility well, I think Perl 5 has done a decent job. Fifteen year old code bases is at least within the realm of possibility to get up and running, and the language has evolved quite a bit in the meantime.

[antod]

Yeah I remember occasions when PHP x.y.z patch releases would break existing code.

Sure it can be possible to run PHP4 code on PHP5, but only AFTER you have tested and debugged it on the newer PHP version.