How do I get the id of the data from previous page?

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PHP

MySQL

HTML5

bootstrap

Hello Everyone! I am new and my english is bad thus also a student learning to code for my project.
I have a problem and I don't know how to get a variable from different php page. Ty guys

The code of form:

<h3>Sell Your Item</h3>
<form method='post' action='createitem.php'>
    <label>Product Name:</label>
    <input type='text' name='productname' class='form-control' required>
    <br>
    <label>Product Type:</label>
    <select class="form-select" aria-label="Default select example" name='producttype' required>
        <option selected>Open this select menu</option>
        <option value="Makeup tools">Makeup tools</option>
        <option value="Makeup">Makeup</option>
        <option value="Trending clothes">Trending clothes</option>
        <option value="Woman">Women</option>
        <option value="Men">Men</option>
        <option value="Kids">Kids</option>
        <option value="Shoes">Shoes</option>
        <option value="Bags">Bags</option>
        <option value="Kitchen wares">Kitchen wares</option>
        <option value="Home appliances">Home appliances</option>
        <option value="Hello kitty items">Hello kitty items</option>
        <option value="Skincare">Skincare</option>
    </select>
    <br>
    <label>Product Description:</label>
    <textarea class="form-control" name="productdescription" rows="5"></textarea>
    <br>
    <label>Product Price:</label>
    <input type='number' name='productprice' class='form-control' required>
    <br>
    <label>Product Image:</label>
    <input type='text' name='productimg' class='form-control' placeholder="Please enter a valid image url" required>
    <br>
    <input type='submit' class='btn btn-primary' name='add' id='add' value='Sell Item'>
</form>

Here is where form is inserted to sqli tables:

<?php
	session_start();
	ob_start();

    include 'dbconnection.php';
		
		if(!isset($_SESSION['id'])){
			header("Location: login.php"); 
            exit();
		}
    
    $productname = $_POST['productname'];
    $producttype = $_POST['producttype'];
    $productdescription = $_POST['productdescription'];
	$productprice = $_POST['productprice'];
	$productimage = $_POST['productimg'];

    $query = "INSERT INTO products (productname,producttype,productdescription,productprice,productimg) VALUES
    ('$productname','$producttype','$productdescription','$productprice','$productimage')";?>

And here is where I want to insert the id:

<pre>
<?php
    include "dbconnection.php";
    
    $query = "SELECT * FROM products where productid = 32'"; // the number 32 is the placeholder for the inserting value
    $result = mysqli_query($conn, $query);
    $checkproducts = mysqli_num_rows($result) > 0;
    
    if($checkproducts){
        while($row = mysqli_fetch_array($result)){
    ?>

What I have tried:

I have tried some methods like last insert and I can't seem to make it work or maybe I
just don't really know. Sorry for inconvinience.