If I were to transition to a Mac, what should I expect with regard to programming for the web? Where should I expect to see major changes?
The biggest change would be if you're using PHP to open local files with absolute path:
On OS X, you will use the format similar to
/home/jdelage/workspace/web/ instead of
C:\users\jdelage\workspace\web. Make sure you update all your code to forward slashes instead of backslashes - forward slashes work on both platorms
Installing optional PHP modules will be a little different, but should be easier on OS X.
There are few other corner case items items but remember that PHP is designed to be cross-platform, and tries hard to abstract the underlying OS away from you.