An actual debugger. I started as a PHP/WP dev and spent many hours running results through echo or var_dump. IMO the debugger is the absolute first thing you need to learn about the platform you're writing for. Without it, you're taking shots in the dark and you truly don't know how your code is executing.

It seriously pains me to see people not using one. I have a friend who is taking an online PHP backend class. There was one lecture on debugging, and all it consisted of was "here's what using var_dump looks like". I showed my friend how to actually set breakpoints in their JS code, set watches, etc and they felt cheated by their class. They should.