I've slowly been learning PHP the last 6 months or so, and finally spent a good few weeks hacking this backlink discover tool together. It's kinda the tool I always wanted for PR, SEO. It's just the first version, lots of improvements needed!
No source code, but the tool uses a number of different APIs, some paid, some free. The Moz API, which has a free tier has a good repo for getting started here: https://github.com/seomoz/SEOmozAPISamples