Simple PHP contact form with Firebase hosting

From the Firebase Hosting site (emphasis mine):

We deliver all of your static content (html, js, images, etc.) over a secure SSL connection and serve it on a CDN.

Firebase Hosting is for hosting static assets. Firebase currently doesn't offer any way to execute your code on Firebase's servers.

Update (2018-08-08): You can now run Node.js/JavaScript code but connecting your Firebase Hosting project to Cloud Functions + Firebase Hosting. But that still won't allow you to run PHP code.

As per the latest update firebase has started using Cloud Functions

Cloud Functions for Firebase lets you run mobile backend code that automatically responds to events triggered by Firebase features and HTTPS requests. Your code is stored in Google’s cloud and runs in a managed environment. There's no need to manage and scale your own servers.

For more : https://firebase.google.com/docs/functions/