While building Shift Pic, I needed to interact with FastSpring's API to validate licenses and subscriptions. After doing some research, I found that FastSpring did not have an official PHP library for their API.
I got to work, and I published this unofficial FastSpring API library 2-3 hours later. It's not perfect, nor is it 100% complete, but it's a great starting point that I will likely develop some more in the feature if other developers have a need for more features.
The library is available as a Composer package - more information is available on the GitHub repository's page.