Implementing web compilation of an Android app

I'm trying to run a script in a php file that will run the "gradlew assembleDebug" command on clicking a button. The problem I'm encountering is that the script definitely runs but the build always fails. If I run the same script through my terminal, it works.

<?php `/opt/android-project/gradlew assembleDebug`  ?>

That's what my script contains. It assumes that the users project is in /opt/android-project

All I get as the error is BUILD FAILED