I need to nest 2 arrays so that I can echo order h

I have updated my original post based on what I learned from your comments below. It is a much simpler process than I originally thought.

    require '../database.php';

    $pdo = Database::connect();
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    $sql = "SELECT * FROM Orders WHERE id = 430";
            $q = $pdo->prepare($sql);
            $q->execute(array($id));
            $data = $q->fetch(PDO::FETCH_ASSOC);

            echo    'Order Num: ' . $data['id'] . '<br>';

    $sql = "SELECT  * FROM Order_items 
            JOIN    Parts ON Parts.id = Order_Items.part_id 
            WHERE   Order_Items.orders_id = 430";

            $q = $pdo->prepare($sql);
            $q->execute(array($line_item_id));
            $data = $q->fetch(PDO::FETCH_ASSOC);

            while ($data = $q->fetch(PDO::FETCH_ASSOC)) 
            {
                echo '- ' . $data['part_num'] . $data['qty'] . "<br>";
            }

     Database::disconnect();

Unfortunately, only my first query is producing results. The second query is producing the following ERROR LOG: "Base table or view not found: 1146 Table 'Order_items' doesn't exist" but I am expecting the following results.

Expected Results from Query 1: Order Num: 430

Expected Results from Query 2:

- Screws 400
- Plates 35
- Clips 37
- Poles 7
- Zip ties 45

Now that I understand where you are coming from, let's explain a couple of things.

1.PDO and mysqli are two ways of accessing the database; they essentially do the same things, but the notation is different.

2.Arrays are variables with multiple "compartments". Most typical array has the compartments identified by a numerical index, like:

$array[0] = 'OR12345'; //order number
$array[1] = '2017-03-15'; //order date
$array[2] = 23; //id of a person/customer placing the order

etc. But this would require us to remember which number index means what. So in PHP there are associative arrays, which allow using text strings as indexes, and are used for fetching SQL query results.

3.The statement

$data = $q->fetch(PDO::FETCH_ASSOC)

$row = $result->fetch_assoc()

do exactly the same thing: put a record (row) from a query into an array, using field names as indexes. This way it's easy to use the data, because you can use field names (with a little bit around them) for displaying or manipulating the field values.

4.The

while ($row = $result->fetch_assoc())

does two things. It checks if there is a row still to fetch from the query results. and while there is one - it puts it into the array $row for you to use, and repeats (all the stuff between { and }).

So you fetch the row, display the results in whatever form you want, and then loop to fetch another row. If there are no more rows to fetch - the loop ends.

5.You should avoid using commas in the FROM clause in a query. This notation can be used only if the fields joining the tables are obvious (named the same), but it is bad practice anyway. The joins between tables should be specified explicitly. In the first query you want the header only, and there is no additional table needed in your example, so you should have just

SELECT *
FROM Orders
WHERE Orders.Order_ID = 12345

whereas in the second query I understand you have a table Parts, which contains descriptions of various parts that can be ordered? If so, then the second query should have:

SELECT *
FROM Order_items
JOIN Parts ON Parts.ID = Order_Items.Part_ID
WHEERE Order_Items.Order_ID = 12345

If in your Orders table you had a field for the ID of the supplier Supplier_ID, pointing to a Suppliers table, and an ID of the person placing the order Customer_ID, pointing to a Customers table, then the first query would look like this:

SELECT * 
FROM Orders
JOIN Suppliers ON Suppliers.ID = Orders.Supplier_ID
JOIN Customers ON Customers.ID = Orders.Customer_ID
WHERE Orders.Order_ID = 12345

Hope this is enough for you to learn further on your own :).