How can I connect php to mysql linux database?

This is the story I got a linux server  that runs on my rasbarri pi and I installed on an ubuntu server .I installed mysql using sudo apt install mysql-server.I created a basic database and I tried to connect to it using this code.I installed php using sudo apt install php

<?php
$servername = "localhost";
$username = "username";
$password = "password";

// Create connection
$conn = new mysqli($servername, $username, $password);

// Check connection
if (mysqli_connect_error()) {
  die("Database connection failed: " . mysqli_connect_error());
}
echo "Connected successfully";
?>

I thinked the probelm is becaus of sudo permission so I disabled sudo because I tought is because of it.It didn't work.It doesn't display that it connected to the databse.What is the problem in my code or where did I go wrong?.All the credentials are corect but it doesn't confirm that I am connected.Do not tell me to change to Javscript or download some weird files.I am not dumb.Is a problem with the php code?If yes where?Or is a problem with how I configured the server?