I think their object model is a good example. Introduced OOP in version 4 but they implemented it in a way that was unusable. They put it right in version 5 but that broke older code -- and it was still weird. It was only in the latest release that it became possible that you could get the right class name in a static method of a class that was inherited by another. Everything they do, they do wrong[1] and it takes them years to get it into approximative acceptable shape.

The only thing that saves them is that most php users have no clue of what they are doing, have not training of any sorts, and no nothing but php and maybe perl, vb or whatever.

[1] Okay, I'm exaggerating and the latest iteration is no longer the menace it once used to be.