Android Question How to call / trigger a sub from php

Hi, I would like to make an app that sends sms messages when the php requests so. The best I figured was to put a timer that checks mysql database in search of new messages to be sent. And when it sends the sms from the database, then it writes 1 in a field to mark it as sent. But I will be using a lot of time and resources if I check the whole database every 5 seconds even if there are no changes, the same goes with html pages. Is there some way to parse some data from web server to app itself without the app actively checking for the data, but rather as a "post" to the app?

Thank you so much for response