android应用与webservice的通信安全

I am developing an android app for a site, which I made a capability in it that users can send data to the site database in the app. I use PHP web services and URL in android code for connecting to the web service. how can I make my app (and my php web service) secure that no one can't find the web service url by debugging my apk and send amount of data to the site and make the site down.I am developing an android app for a site, whic