Instead of converting (which would lead to an unmanageable spaghetti code), make the PHP project a regular .NET project (like F# or C++/CLI) using PHP compiler we're working on:
https://github.com/iolevel/peachpie
In this way, beside other things, you keep your PHP code maintainable while you can seamlessly use it from C#.
Some resources:
- http://www.peachpie.io/
- https://youtu.be/xnQTsdJhvtk