foreach ($product_id as $index => $h) {
    $v = $size_id[$index];
    $q = $quantity[$index];
    $insert_pending = "INSERT INTO pending_orders 
                            (cus_id,product_id,size_id,quantity,ip_address) VALUES 
                            ('$cus_id','$h','$v','$q','$ip_address')";
    $run2 = mysqli_query($con, $insert_pending);
}