free development tool for developing php dynamic website

It's not clear what could be "not-dynamic" if you use PHP. Anyway, everything (or maybe nearly everything) related to PHP is open-source and free of charge. The HTTP server supporting PHP also can be open-source and free. First and foremost, this is Apache. If you get some paid hosting service, you pay for service, using company resources, support, and not for software.

In contrast to that, development using APS.NET is based on proprietary software, but it also can be achieved free of charge (in terms of software purchase or fee), depending on your project, size of the team and other factors. Carefully read the licensing terms for the products you try to use. You can even use Microsoft Visual Studio free of charge, if it is Community Edition. Nevertheless, it is fully functional. Again, check the license agreements. Alternatively, you can use open-source products for development, such as SharpDevelop or MonoDevelop.

—SA