Is it any worse than Javascript, if we're honest?
There's seriously wonky things in there like the associativity on ?: being wrong, and == being even more broken than in js. There's order of arguments on array_map and array_filter being inconsistent. It's not that there's a massive flaw in there that kills it, it's death by a thousand paper cuts.
Threading/asynchronicity comes to mind. I've worked on a few projects that started out as a web-front end, but then needed to do some background processing. Eventually the background process needs to be multi-threaded and the choices are: rewrite your code to support pthread's wrappers classes, use fork and shared memory segments, asynchronous web requests. All those solutions work, but they are much more complicated than the same task would be in most other languages with native threading support.
I find it quite sad that most PHP developers seem to think that writing code that looks more like Java is the "correct" thing when its greatest features have always allowed loose types and first class functions. After all that's what makes it so easy to work with?
Perhaps PHP could have been more like Scala a few years back already.. the perfect combination of Objects and Functions...
The real annoying part is that by nature it will never have the same amount of IDE niceties you'd get with more typed languages
I understand JS having its place as being ubiquitously supported in browsers but, just as PHP, it's bad and its problems are not about niceties from IDEs or static typing.
It's not quite bad enough to count as a Turing tarpit like INTERCAL or brainf*ck, but it's one of the closest mainstream languages to the tarpit.