Doubt of connection string between azure and php

Greetings, I have a question:

I have a mysql database in azure and I want to connect it with php but I do not know how. I have this command but I do not know why to replace where it says "localhost" and $ db. Thanks in advance.

<?php
    $user = "root";
    $pass = "";
    $db = "db";

    $db = new mysqli("localhost", $user, $pass, $db) or die("Unable to connect");

    echo"Great work!"

?>