получение предупреждения myspli при доступе к странице списка инвентаря

Question

Я создаю страницу списка инвентаря, и когда я пытаюсь получить к ней доступ, я получаю предупреждение:

Предупреждение: mysqli_num_rows () ожидает, что параметр 1 будет mysqli_result, строка указана в C: \ xampp \ htdocs \ pinkys_pearls \ storeadmin \ inventory_list.php в строке 102.

Может кто-нибудь сказать мне, что это за ошибка и как ее исправить.

Вот код, который я использую:

    <?php

session_start();

if(isset($_POST['button'])) {

    include_once("../storescripts/connect_to_mysql.php");
    $con = mysqli_connect("$db_host","$db_username","$db_pass","$db_name");
    $username = mysqli_real_escape_string($con, $_POST['username']);
    $password = mysqli_real_escape_string($con, $_POST['password']);

    //Error handler
    //Check for empty fields
    if (empty($username) || empty($password)) {
        header("Location: ../admin_login.php?admin_login=empty");
        exit();
    } else {
        //Check if charactors are valid
        if (!preg_match("/^[a-zA-Z0-9]*$/", $username) || !preg_match("/^[a-zA-Z0-9]*$/", $password)) {
            header("Location: ../admin_login.php?admin_login=invalid");
        exit();
        } else {
            $sql = "SELECT * FROM admin WHERE username = '$username'' AND password = '$password'";
            $result = mysqli_query($con, $sql);
            $resultCheck = mysqli_num_rows($result);
            if ($resultCheck < 1) {
                header("Location: ../admin_login.php?admin_login=invalid");
                exit();
            } else {
                if ($row = mysqli_fetch_assoc($result)) {

                    $_SESSION['manager'] = $row['username'];
                    $_SESSION['manager_pwd'] = $row['password'];
                    header("Location: admin_index.php"); //relocate to index page
                    exit();
                } else{
                    echo 'username and password invalid. Please try again';
                }
            }
        }
    }

}
?>
<?php 
// Script Error Reporting
error_reporting(E_ALL);
ini_set('display_errors', '1');
?>
<?php 
// Delete Item Question to Admin, and Delete Product if they choose
if (isset($_GET['deleteid'])) {
    echo 'Do you really want to delete product with ID of ' . $_GET['deleteid'] . '? <a href="inventory_list.php?yesdelete=' . $_GET['deleteid'] . '">Yes</a> | <a href="inventory_list.php">No</a>';
    exit();
}
if (isset($_GET['yesdelete'])) {
    // remove item from system and delete its picture
    // delete from database
    $id_to_delete = $_GET['yesdelete'];
    $result = mysqli_query("DELETE FROM products WHERE id='$id_to_delete' LIMIT 1") or die (mysqli_error());
    // unlink the image from server
    // Remove The Pic -------------------------------------------
    $pictodelete = ("inventory_images/$id_to_delete.jpg");
    if (file_exists($pictodelete)) {
                unlink($pictodelete);
    }
    header("location: inventory_list.php"); 
    exit();
}
?>
<?php 
// Parse the form data and add inventory item to the system
if (isset($_POST['item_number'])) {

    $item_number = mysqli_real_escape_string($_POST['item_number']);
    $price = mysqli_real_escape_string($_POST['price']);
    $category = mysqli_real_escape_string($_POST['category']);
    $subcategory = mysqli_real_escape_string($_POST['subcategory']);
    $description = mysqli_real_escape_string($_POST['dscription']);
    // See if that product name is an identical match to another product in the system
    $sql = mysqli_query("SELECT id FROM products WHERE item_number='$item_number' LIMIT 1");
    $productMatch = mysql_num_rows($sql); // count the output amount
    if ($productMatch > 0) {
        echo 'Sorry you tried to place a duplicate "item_number" into the system, <a href="inventory_list.php">click here</a>';
        exit();
    }
    // Add this product into the database now
    $sql = mysqli_query("INSERT INTO products (item_number, price, description, category, date_added) 
        VALUES('$item_number','$price','$description','$category',now())") or die (mysql_error());
     $pid = mysqli_insert_id();
    // Place image in the folder 
    $newname = "$pid.jpg";
    move_uploaded_file( $_FILES['fileField']['tmp_name'], "inventory_items/$newname");
    header("location: inventory_list.php"); 
    exit();
}
?>
<?php 
// This block grabs the whole list for viewing
$product_list = "";
$sql = "SELECT * FROM products ORDER BY date_added DESC";
$productCount = mysqli_num_rows($sql); // count the output amount
if ($productCount > 0) {
    while($row = mysqli_fetch_array($sql)){ 
             $id = $row["id"];
             $item_number = $row["item_number"];
             $price = $row["price"];
             $date_added = strftime("%b %d, %Y", strtotime($row["date_added"]));
             $product_list .= "Product ID: $id - <strong>$product_name</strong> - $$price - <em>Added $date_added</em> &nbsp; &nbsp; &nbsp; <a href='inventory_edit.php?pid=$id'>edit</a> &bull; <a href='inventory_list.php?deleteid=$id'>delete</a><br />";
             $qty = $row["qty"];
    }
} else {
    $product_list = "You have no products listed in your store yet";
}
?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Inventory List</title>
<link rel="stylesheet" href="../style.css" type="text/css" media="screen" />
</head>

<body>
<div align="center" id="mainWrapper">
  <div id="pageContent"><br />
    <div align="right" style="margin-right:32px;"><a href="inventory_list.php#inventoryForm">+ Add New Inventory Item</a></div>
<div align="left" style="margin-left:24px;">
      <h2>Inventory list</h2>
      <?php echo $product_list; ?>
    </div>
    <hr />
    <a name="inventoryForm" id="inventoryForm"></a>
    <h3>
    &darr; Add New Inventory Item Form &darr;
    </h3>
    <form action="inventory_list.php" enctype="multipart/form-data" name="myForm" id="myform" method="post">
    <table width="90%" border="0" cellspacing="0" cellpadding="6">
      <tr>
        <td width="20%" align="right">Item Number</td>
        <td width="80%"><label>
          <input name="item_number" type="text" id="item_number" size="64" />
        </label></td>
      </tr>
      <tr>
        <td align="right">Product Price</td>
        <td><label>
          $
          <input name="price" type="text" id="price" size="12" />
        </label></td>
      </tr>
      <tr>
        <td align="right">Category</td>
        <td><label>
          <select name="category" id="category">
          <option value="Bracelets">Bracelet</option>
          <option value="Necklace">Necklace</option>
          <option value="Earring">Earring</option>
          <option value="Childrens">Childrens</option>
          <option value="Sets">Sets</option>
          <option value="Rosary">Rosary</option>
          <option value="Accessories">Accessories</option>
          </select>
        </label>
      </tr>
      <tr>
        <td align="right">Description</td>
        <td><label>
          <textarea name="description" id="description" cols="64" rows="20"></textarea>
        </label></td>
      </tr>
      <tr>
        <td align="right">Product Image</td>
        <td><label>
          <input type="file" name="fileField" id="fileField" />
        </label></td>
      </tr>      
      <tr>
        <td>&nbsp;</td>
        <td><label>
          <input type="submit" name="button" id="button" value="Submit" />
        </label></td>
      </tr>
    </table>
    </form>
    <br />
  <br />
  </div>
</div>
</body>
</html>