Nearly every PHP developer I know

XDebug

Holy shit, that's my usual method of debugging. Which is also why I get called in when other devs don't find the bug because shit doesn't show up in the graphic debugger. Like hardware/software race conditions. I had one of these that was so nasty that it took me a full week of staring at the code - but finally, it DID confess.

When colleagues asked how I found it, I even told them it was "Chuck Norris debugging". :-)

@CodeMonkeyG that's what I tell them on a regular basis, again and again. They do not listen...

@Fast-Nop haha nice one!
I think one should know both method of debugging well. Sometimes a little interactive shell session teaches me more than any debugging. (In JS or C# for example)