Behöver hjälp med php

<?php
include("header.php");

$sql=mysqli_query($link, "SELECT * FROM article");
$sql_num=mysqli_num_rows($sql);
echo $sql_num;

?>
<?php
include("footer.php");
?>

Notice: Undefined variable: link in C:\xampp\htdocs\dyn\articles.php on line 4

Warning: mysqli_query() expects parameter 1 to be mysqli, null given in C:\xampp\htdocs\dyn\articles.php on line 4

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, null given in C:\xampp\htdocs\dyn\articles.php on line 5

<?php
$db_host = "localhost";
  $db_username = "root";
  $db_password = "";
  $db_database = "phptest";
$link = mysqli_connect($db_host,$db_username,$db_password) or die("Cannot connect");
  mysqli_select_db($link, $db_database) or die("Cannot select database");
  ?>