PHP MySQL Update for Procedural Connection

<?php
  $server = 'host';
  $user = 'user'; 
  $pass = 'pass'; 
  $db = 'db';
  // Create connection  
  $conn = mysqli_connect($server, $user, $pass, $db);  	
  // Check connection  	
  if (!$conn) { 
    die("Could not connect: " . mysqli_connect_error());
  }
  $sql = "UPDATE users SET username='john.doe' WHERE user_id=2";  
  if (mysqli_query($conn, $sql)) {
    echo "Update successful.";
  } else {
    echo "Could not update: " . mysqli_error($conn);
  }
  mysqli_close($conn);  
?>