Variables in PHP are non-typed. A variable can thus hold any possible type: an integer, string, a floating point number, and even an object or an array. C++ on the other hand is a typed language. In C++ an integer variable always has a numeric value, and a string variable always hold a string value.
When you mix native code and PHP code, you will need to convert the non-typed PHP variables into native variables, and the other way round: convert native variables back into non-typed PHP variables. The PHP-CPP library offers the "Value" class that makes this a very simple task.