How does the debugger "know" where to get the debug source?

I was having problems with PhpStorm not debugging my code, and thought looking at phpinfo() might help.

Ironically when I tried to run a phpinfo() script in my browser it jumped into the debugger. :^O

It seems that the debugger knew where to find that script, but not my main application source.

I'm using IIS on my dev machine, and one difference is that the phpinfo() script was in my c:\web directory, which is my root for localhost.

My application is in a separate directory under c:\dev (e.g. c:\dev\app\) which is set up as a virtual directory in IIS.

I figure that either XDebug or PHPStorm needs to be told this somehow, just not sure where.

Any help apprecated.