尝试登录我的网站管理页面时收到错误消息

Question

我使用教程创建了一个管理页面。当我尝试登录我的数据库时，我收到错误消息，例如：

  

致命错误。未定义的函数mysql_fetch_array第22行。

当我尝试修复此问题时，我会看到空白页面。我知道我必须将所有内容升级到mysqli。我无法确定错误的位置。任何想法？

<?php 
session_start();
if (isset($_SESSION["username"])) {
    header("admin_index: admin.php"); 
exit();
}
?>
<?php
// Parse the log in form if the user has filled it out and pressed "Log In"
if (isset($_POST["username"]) && isset($_POST["password"])) {

    $username = trim($_POST["username"]); // filter everything but numbers 
   and letters
    $password = trim($_POST["password"]); // filter everything but numbers 
    and letters
    // Connect to the MySQL database  
include "../storescripts/connect_to_mysql.php"; 
$link = mysqli_connect("$db_host","$db_username","$db_pass","$db_name");
$result = mysqli_query($link,"SELECT username FROM admin WHERE 
   username='$username' AND password='$password' LIMIT 1"); // query the 
   person

// ------- MAKE SURE PERSON EXISTS IN DATABASE ---------
$existCount = mysqli_num_rows($result); // count the row nums
if ($existCount == 1) { // evaluate the count
     while($row = mysql_fetch_array($result)){ 
         $id = $row["id"];
     }
     $_SESSION["id"] = $id;
     $_SESSION["username"] = $username;
     $_SESSION["password"] = $password;
     header("location: admin_index.php");
     exit();
} else {
    echo 'That information is incorrect, try again <a href="index.php">Click 
Here</a>';
    exit();
    }
}
?>