I am developing an application for the Android that requires a connection with PHP pages in order to add sensitive data to a database that will affect the application. Since it's very easy to reverse engineer an android app, one can simply find the url where the data is sent to and manipulate it. I thought about creating a registration based on IMEI, but one can still able to manipulate it for his malicious purposes. I have also checked OAuth but I didn't really understand how it works and if it can help in my condition. What can I do to fully secure my application? Thanks in advance!

EDIT: By the way, what I am mostly trying to achieve here is to make sure the requests are being sent from an Android and not from any other device.