Updating Data With PHP MySQL

<?php
  $server = 'host'; 
  $user = 'user'; 
  $pass = 'pass'; 
  $db = 'db';
  // Create connection
  $conn = new mysqli($host, $user, $pass, $db);  	
  // Check connection  	
  if ($conn->connect_error) {
      die("Could not connect: " . $conn->connect_error);
  }
  $sql = "UPDATE users SET username='john.doe' WHERE user_id=2";
  if ($conn->query($sql) === TRUE) {
    echo "Update successful.";
  } else {
    echo "Could not update: " . $conn->error;
  }
  $conn->close();  
?>