Hi,
First of all I'm not an expert on PHP, but we have done the integration. As I remember, we connect to the DI API through our customized DLL (another layer). You need to register that DLL in PHP (That DLL should be available to PHP, you may know how to do that).
So that extra layer will expose the DI API to your PHP code. There might be a way to access the DI API directly.
In both ways, I think you should register those DLL in PHP to use.
Hope this tip may help, Unfortunately, I don't have the code with me now.
Regards,
J S L Geeganage