Spark is one that I was going to suggest - I can't say I've personally used it, but it was something I certainly evaluated before going down a different route.

Spark is one that I was going to suggest - I can't say I've personally used it, but it was something I certainly evaluated before going down a different route. From the outside, it looks like an excellent basis for a SaaS built upon PHP and the Laravel framework. I personally made the age-old mistake of telling myself that it would be cool to build something in a new language as some form of personal development. Of course, the idea grew legs and now I'm learning as I go. Not strictly a problem, as I'm loving Django and Python, just an extra hurdle to scale before launching.