Developing a native mobile application on ios and Android with a lot of web interfacing. What are my best options for web backend storage and code?

I am currently developing my web app that will support my application in php. We frequently run the http client in the native os to call upon URLs to run the scripts I have made for the web app. These photos scripts take arguments inside the URL string. I return json data to the client side device. I am wondering what I should migrate to that will assure great scalability and reliability. Also ease of use when implementing the web interfacing in the client side code. I am so busy with getting the application done to meet a deadline that I cannot research best practices. We will have a small grace period of "proof of concept" before we start scaling. During that grace period I want to completely re write the web app to a more robust system. As of now I am using php scripts…