&gt; PHP [...] at least allows you to run 4 code on the 5 interpreter with no issues<p>Speaking as someone who at one time made a living porting PHP4 code to PHP5, that&#x27;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.<p>Guess why all cheap web hosting companies only offer ancient versions of PHP? &quot;I downloaded $crap from teh Internet and it doesn&#x27;t work&quot; is nobody&#x27;s favourite idea of customer support.<p>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.