I am not sure if this is the right place to post this but will continue anyways since I am working on a PHP application. This question is more of an architecture question.

I am working on a web application (PHP). Another developer is
working on a mobile app version for iPhone and Android. The functional requirements of the applications requires allowing one user using the web application (say on desktop) to chat in real time with another user using the mobile app.

I don't have any idea about mobile app development but from what I can think of is the user on the mobile will get chat updates through push notifications. As soon as the notification is clicked, the app will open and the user will then start chatting the other user who is using the desktop version.

So, this is basically a chat functionality. Due to lack of experience I am not sure how can I decide the architecture of my web application. I don't expect a direct answer. It will be great if someone can at least provide me some online resources that can help me decide the architecture and know the right path to take. I googled but I think I am not using the right keywords.

Any help will be appreciated.