Problem with session in Custom PHP Admin Panel

Hello, I am having trouble with this custom admin panel. It is pretty simple, it has an admin login page, an index page , and a page where you can Add and Delete invetory items ( <--this one is the one with the problem ) Everything was working okey, I added 2 items, deleted one, and now it gives me this Error everytime i try to add or deleted any items.


" Your login session data is not on record in the database. "


You can try it out. the URL is http://regismartinez.com/pepe/storeadmin/admin_login.php


The credentials are: administrator / asdf1234


Here is the code for admin_login.php (works fine )

<?php 


session_start();
ob_start();
if (isset($_SESSION["manager"])) {
    header("location: index.php"); 
    exit();
    ob_end_flush();
}
?>


<?php 
// Parse the log in form if the user has filled it out and pressed "Log In"
if (isset($_POST["username"]) && isset($_POST["password"])) {


    $manager = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["username"]); // filter everything but numbers and letters
    $password = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["password"]); // filter everything but numbers and letters
    // Connect to the MySQL database  
    include "../storescripts/connect_to_mysql.php"; 
    $sql = mysql_query("SELECT id FROM admin WHERE username='$manager' AND password='$password' LIMIT 1"); // query the person


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

Code (markup):

Here is the code for the index.php (the admin index page)

<?php 


session_start();
if (!isset($_SESSION["manager"])) {
    header("location: admin_login.php"); 
    exit();
}
// Be sure to check that this manager SESSION value is in fact in the database
$managerID = preg_replace('#[^0-9]#i', '', $_SESSION["id"]); // filter everything but numbers and letters


$manager = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["manager"]); // filter everything but numbers and letters


$password = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["password"]); // filter everything but numbers and letters


// Run mySQL query to be sure that this person is an admin and that their password session var equals the database information
// Connect to the MySQL database  
include "../storescripts/connect_to_mysql.php"; 
$sql = mysql_query("SELECT * FROM admin WHERE id='$managerID' AND username='$manager' AND password='$password' LIMIT 1"); // query the person


// ------- MAKE SURE PERSON EXISTS IN DATABASE ---------
$existCount = mysql_num_rows($sql); // count the row nums
if ($existCount == 0) { // evaluate the count
     echo "Your login session data is not on record in the database.";
     exit();
}
?>

Code (markup):

And here is the code for the inventory_list.php ( the one with the problem I believe)

<?php 


session_start();
if (!isset($_SESSION["manager"])) {
    header("location: admin_login.php"); 
    exit();
}


// Be sure to check that this manager SESSION value is in fact in the database
$managerID = preg_replace('#[^0-9]#i', '', $_SESSION["id"]); // filter everything but numbers and letters


$manager = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["manager"]); // filter everything but numbers and letters


$password = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["password"]); // filter everything but numbers and letters


// Run mySQL query to be sure that this person is an admin and that their password session var equals the database information
// Connect to the MySQL database  
include "../storescripts/connect_to_mysql.php"; 
$sql = mysql_query("SELECT * FROM admin WHERE id='$managerID' AND username='$manager' AND password='$password' LIMIT 1"); // query the person


// ------- MAKE SURE PERSON EXISTS IN DATABASE ---------
$existCount = mysql_num_rows($sql); // count the row nums
if ($existCount == 0) { // evaluate the count
     echo "Your login session data is not on record in the database.";
     exit();
}
?>


<?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'];
    $sql = mysql_query("DELETE FROM products WHERE id='$id_to_delete' LIMIT 1") or die (mysql_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['product_name'])) {
    
    $product_name = mysql_real_escape_string($_POST['product_name']);
    $price = mysql_real_escape_string($_POST['price']);
    $category = mysql_real_escape_string($_POST['category']);
    $subcategory = mysql_real_escape_string($_POST['subcategory']);
    $details = mysql_real_escape_string($_POST['details']);


    // See if that product name is an identical match to another product in the system


    $sql = mysql_query("SELECT id FROM products WHERE product_name='$product_name' LIMIT 1");
    $productMatch = mysql_num_rows($sql); // count the output amount
    if ($productMatch > 0) {
        echo 'Sorry you tried to place a duplicate "Product Name" into the system, <a href="inventory_list.php">click here</a>';
        exit();
    }


    // Add this product into the database now


    $sql = mysql_query("INSERT INTO products (product_name, price, details, category, subcategory, date_added) 
        VALUES('$product_name','$price','$details','$category','$subcategory',now())") or die (mysql_error());
     $pid = mysql_insert_id();
    // Place image in the folder 
    $newname = "$pid.jpg";
    move_uploaded_file( $_FILES['fileField']['tmp_name'], "../inventory_images/$newname");
    header("location: inventory_list.php"); 
    exit();
}
?>




<?php 
// This block grabs the whole list for viewing
$product_list = "";
$sql = mysql_query("SELECT * FROM products ORDER BY date_added DESC");
$productCount = mysql_num_rows($sql); // count the output amount
if ($productCount > 0) {
    while($row = mysql_fetch_array($sql)){ 
             $id = $row["id"];
             $product_name = $row["product_name"];
             $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 />";
    }
} else {
    $product_list = "You have no products listed in your store yet";
}
?>

Code (markup):

This is a tutorial I am trying to follow on youtube. I am new to php , and i do not own this code. However i am stuck with this problem. Thank you in advance for any help you guys can provide me.