Certainly gotchas are a drain on productivity. But I don't think this applies more to PHP than any other language I've used. The fact is, if you are less familiar with the language then you won't be as productive.

I personally spend a lot of time hopping among technologies and I always lean heavily on documentation. PHP has pretty good documentation, and this helps a lot in my opinion. I would guess that a lot of developers don't read the docs and just assume that something in a less-familiar language will work exactly like they expect it to, and then call this a "gotcha" when it doesn't.