Your offer is to build an MVP and I guess it's reasonable to assume that further development of the application happens without you. Sure, an MVP is an MVP but

Your offer is to build an MVP and I guess it's reasonable to assume that further development of the application happens without you. Sure, an MVP is an MVP but that doesn't mean it has to be thrown away. You're using your own "mini framework" which I don't think is fair to your clients. Anyone coming in after you will have to work with this "mini framework" regardless of how good or bad it is. There are so many well constructed and documented frameworks out there (+community) for PHP already available that it's almost a crime not to make use of them. If you build an MVP with a popular framework, e.g. Symfony or Laravel, then this can be used as a feature in selling your service in that your clients will not be stuck with some undocumented bespoke framework but instead they can hire a developer who already has expertise with the framework.