Software to control payments of my sales


I’m selling a product directly to my clients. They pay me a fraction every week until they pay the total amount.

Is there any simple solution, web-based, open source, to keep a record and track those payments ? (I don’t need an ERP!)

I’m a php developer, but I don’t want to waste time reinventing the wheel.

LOL I thought of a weekly calendar event. When you sell an item, create a weekly event for the whole period of payment.