Calling a GET variable in a POST function

I wanted to add the $item_id into the ‘bid’ table after clicking on the button.
But the $item_id from the $_GET cannot be passed into the $_POST function.
Is it incorrect to call a variable like this?

Thank you in advance for your help!

<?php 
include ('config/dbfunctions.php'); 
include ("helpers/validateBid.php");

$user_id = "";
$item_id = "";
$table= 'bid';
$allBids = selectAll($table);
$items = selectAll('item');

if (isset($_GET['item_id'])) {
    $itemInfoPost  = selectOne('item', ['item_id' => $_GET['item_id']]);
    $item_id = $itemInfoPost['item_id'];
  } 
  

if(isset($_POST['bid-btn'])){
  
        unset($_POST['bid-btn']);
        $_POST['user_id'] = $_SESSION['user_id'];
        $_POST['item_id'] = $item_id;   //$item_id cannot be called
        $bid_id = create($table, $_POST);   
              
        header('location: itemInfo.php?item_id='."$item_id"); 
}