It sounds like this company already has a PHP / MySQL infrastructure. Which probably means they have the resources in place to support these technologies. If you are a temporary employee (contractor), then you owe it to your client to leave them something they can maintain. In the end, there is probably no difference in what can be built with either technologies. Only in certain instances would .NET have benefits. If this is a simple website with a db backend, then working with PHP / MySQL is not limiting.