If the Zynga position really comes down to programming PHP all day, don't waste your time on that.
Yes, it might be a game company, but adding a few months of PHP to your resume will add nothing of value.

The Microsoft position sounds much more educational and relevant.
It might not be directly game related, but at least the languages used are close enough.
If you get the option, push for a c++ project instead of a c# project.