title: Firebase Firestore with REST (PHP)
body: <p>I am using Firebase Firestore for one of my mobile application. I need to read something from some external site and update/overwrite one Firestore field every Monday at certain time. Right now I am doing it manually. I want to automate this process.</p>
  
  <p>I have a PHP application with Cpanel. I want to have a CRON job that will update this field every Monday. I am planning to use REST calls to Firestore in order to update the field but not able to do the Authentication part...</p>
  
  <p>I have also created the service account for my application and downloaded the project-name-xxxx.json file with private key. I also have the API-KEY. But not knowing how to use both of these in order to authenticate and communicate. I am trying Postman for now but no luck.</p>
  
  <p>Can some one tells me how can I authenticate with REST API suing the API-KEY and json file I have.</p>
  
  <p>Thanks for your time</p>