On the Zend Developer Zone a new tutorial has been posted looking at taking your pre-existing C++ functionality and wrapping it in a PHP extension to be used directly in your code.
In this tutorial I am going to walk you through creating a PHP extension called "vehicles" which will expose a single class called "Car" (obviously in the real-world, your extensions will expose many classes, but I'm trying to keep things simple). The extension will be built for PHP 5. I am only going to give instructions for building the extension in a UNIX-like environment, although most of what I cover should apply to Windows extension development as well.
The tutorial walks you through everything - how the file layout should look, making a build system, the contents of his files (the sample extension lets you do things to the Car like shift gears and brake) and how to combine the C++ code with the build process to create a shared module you can include right in your php.ini.