I want to build a QA program that will crawl all the pages of a site (all files under a specified domain name), and it will return all external links on the site that doesn't open in a new window (does not have the target="_blank" attribute in the href).

I can make a php or javascript to open external links in new windows or to report all problem links that don't open in new windows of a single page (the same page the script is in) but what I want is for the QA tool to go and search all pages of a website and report back to me what it finds.

This "spidering" is what I have no idea how to do, and am not sure if it's even possible to do with a language like PHP. If it's possible how can I go about it?