I don't know the details of PHP, but it seems to me that all the areas that are referred to as "implementation-dependent" are code-observable: e.g. the deferred array copying decision discussed in the critique: if your code depends on a behavior, you'll get different results on the other platform. So if you want to be portable, you need to sniff the behavior and take a different route on the alternate platform, or you need to avoid the quicksand in the first place.