It's possible to download old source code versions from open source projects. I want to learn from history. Maybe, more people in Stack Overflow want to learn from history too.
What open source projects do you know that made a transition from bad php code to good php code? How do you describe the transition (big, fast, easy...)?
What code smells am I going to see? What best practices am I going to see? Any additional reason(s) to use your example(s)?
You can usually find old releases posted on the download pages. The best source of old code, however, is the source control tool. You can watch code evolve over time using the checkins.
Why not jump into a cvs repository to see individual commits. You can check out the whole drupal code base or clone the cvs repo into git.
You could then get real serious data on commits. You could probably just look through the cvs commits on the drupal site
Usually every open-source project shows (and let download them) all (or all the major) release, as well as the release note for each of them.
Some Examples:
Just go in the download page and look for 'archive releases' or 'other releases'