Error reporting in Textwrangler

I'm starting to play around with some php scripting in Textwrangler on my new macbook with a few tutorials from some books I've found. However, one of the exercises is purposefully creating undefined variables just to so you what the errors look like when you try to load the page. However, when I load the page after saving my document the screen is blank instead of saying some kind of notice of undefined variables on line such and such. How do I turn on some kind of error reporting to know when i goof?

Thanks.