Symfony 2 is quality engineering. It's quite nice just to browse the source.
I think the technical debt of PHP is a huge reason why development of the software behind Wikipedia hasn't fixed any of the huge issues in the last 5 years.
You don't need to convince me that PHP is a clunky, unpleasant language. My point is that there are still legitimate reasons to choose it.
Languages are just tools. Some are better than others for a specific task. This is a fact. You choose the language for a given task because it fits the task better than other options. That's all there is to it.
I don't currently use PHP, but I acknowledge that there are situations where using it makes sense. I'm just not in one of those situations in present. Saying that PHP is a bad choice in every situation is worse than stupid: it's simply wrong.
Its standard library is no more comprehensive than those of comparable languages.
The only advantage left, then, is the same one all the sibling comments bring up: it's already installed on the server.
So the question remains: Why would anyone who has a choice and knows that other languages exist ever use it for anything?
Because that shit gets the job done. Pick the right tool for the job, sometimes PHP is the right tool.
Javascript is a sad story though, it's an abysmal language (even with ES6, though that does make things better) but it enjoys a monopoly over the browser runtime space.
I think asking why anyone would start a new project in PHP is a very reasonable thing to ask.
If you want quick and dirty there is Ruby + Sinatra, Python + Flask or Go + Martini. All of which are safer, have less 'quirks' as you say and are all faster. None of them require any more work than the current PHP alternatives.
PHP may be superior to some languages in some regards, but it's definitely no longer the king of the hill of prototyping languages.