I'm not quite sure I understand what you want. However, you could program your own PHP extension in C as PHP source is written in C.
If the API of your application is written in C (I'm not quite sure about C++), and is contained in a header file you could easily include it in the source code of a PHP extension. You could then use all the functions of the application library to create your own PHP API.
Hope that is what you wanted to know.
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