How to pass data from one page into another page

im trying to pass data from one page into the next but im having problems i don't know where to modify this code po_db.inc

trying to add another add_po from my po_inquiry_items and save them on to another page like pr_entry_items
where can i find button for this function to change from add_po into add_po_inquiry ---> po_inquiry_items ?, how do i pass the value from po_inquiry_items into my -----> pr_entry_items?.

function add_po(&$po_obj)
{
    global $Refs;

    begin_transaction();
    hook_db_prewrite($po_obj, ST_PURCHORDER);

     /*Insert to purchase order header record */
     $sql = "INSERT INTO ".TB_PREF."purch_orders (supplier_id, Comments, ord_date, reference,
         requisition_no, into_stock_location, delivery_address, total, tax_included, prep_amount) VALUES(";
     $sql .= db_escape($po_obj->supplier_id) . "," .
         db_escape($po_obj->Comments) . ",'" .
         date2sql($po_obj->orig_order_date) . "', " .
         db_escape($po_obj->reference) . ", " .
         db_escape($po_obj->supp_ref) . ", " .
         db_escape($po_obj->Location) . ", " .
         db_escape($po_obj->delivery_address) . ", " .
         db_escape($po_obj->get_trans_total()). ", " .
         db_escape($po_obj->tax_included). ", " .
         db_escape($po_obj->prep_amount). ")";

    db_query($sql, "The purchase order header record could not be inserted");

     /*Get the auto increment value of the order number created from the sql above */
     $po_obj->order_no = db_insert_id();

     /*Insert the purchase order detail records */
     foreach ($po_obj->line_items as $line_no => $po_line)
     {
        $sql = "INSERT INTO ".TB_PREF."purch_order_details (order_no, item_code, description, delivery_date,    unit_price,    quantity_ordered) VALUES (";
        $sql .= $po_obj->order_no . ", " . db_escape($po_line->stock_id). "," .
        db_escape($po_line->item_description). ",'" .
        date2sql($po_line->req_del_date) . "'," .
        db_escape($po_line->price) . ", " .
        db_escape($po_line->quantity). ")";
        db_query($sql, "One of the purchase order detail records could not be inserted");
        $po_obj->line_items[$line_no]->po_detail_rec = db_insert_id();
     }

    $Refs->save(ST_PURCHORDER, $po_obj->order_no, $po_obj->reference);

    add_audit_trail(ST_PURCHORDER, $po_obj->order_no, $po_obj->orig_order_date);
    hook_db_postwrite($po_obj, ST_PURCHORDER);
    commit_transaction();

    return $po_obj->order_no;
}