Error working with Sales Quotation

function delete_sales_order($order_no, $trans_type)
{
    begin_transaction();
    hook_db_prevoid($trans_type, $order_no);

    $sql = "DELETE FROM ".TB_PREF."sales_orders WHERE order_no=" . db_escape($order_no) 
        . " AND trans_type=".db_escape($trans_type);

    db_query($sql, "order Header Delete");

    $sql = "DELETE FROM ".TB_PREF."sales_order_details WHERE order_no =" 
        .db_escape($order_no) . " AND trans_type=".db_escape($trans_type);
    db_query($sql, "order Detail Delete");

    delete_reference($trans_type, $order_no);

    add_audit_trail($trans_type, $order_no, Today(), _("Deleted."));
    commit_transaction();
}

function add_audit_trail($trans_type, $trans_no, $trans_date, $descr='')
{
    $sql = "INSERT INTO ".TB_PREF."audit_trail"
        . " (type, trans_no, user, fiscal_year, gl_date, description, gl_seq)
            VALUES(".db_escape($trans_type).", ".db_escape($trans_no).","
            . $_SESSION["wa_current_user"]->user. ","
            . get_company_pref('f_year') .","
            . "'". date2sql($trans_date) ."',"
            . db_escape($descr). ", 0)";

    db_query($sql, "Cannot add audit info");
    
    // all audit records beside latest one should have gl_seq set to NULL
    // to avoid need for subqueries (not existing in MySQL 3) all over the code
    $sql = "UPDATE ".TB_PREF."audit_trail SET gl_seq = NULL"
        . " WHERE type=".db_escape($trans_type)." AND trans_no="
        .db_escape($trans_no)." AND id!=".db_insert_id();

    db_query($sql, "Cannot update audit gl_seq");
}