FWIW, OpenWhisk, while available on IBM Bluemix, is also open source and can be run wherever you so desire: https://github.com/openwhisk/openwhisk
BTW: I really hate the marketing inventions. There's no cloud, or serverless, you still run your software on someone's server.
Anyway the serverless is essentially (as someone here pointed out) the same idea as shared hosting which is very popular with php. It just uses a different name.