Most annoying PHP change, starting in version 7: it now throws warnings if you try to access an array element with an undefined key. This one is making me nuts, because I wrote a lot of code that relied on the old behavior where it just treated it as null, instead of throwing undefined warnings. All of those spots in the code need to check that an array key exists before testing it.

I understand why. They’re trying to impose stronger type-checking on PHP, but it makes updating old code a real bitch.