Do not insert Master record Detail (or point of sale) in my Php code, why suggestions?

I have been programming in Php and MySQL a practice of master detail which has as fields the "id_detalle_renta", "idrenta", "idauto", which inserts the rent, but not the detail of the rent, then the php code with certain parts commented, I would be very helpful, thanks.

<?php 
    session_start();
    require 'bd/conexion_bd.php';


    if(!isset($_SESSION['status_renta']))
    {
        $_SESSION['status_renta']='Cerrada';    
    }

    //echo var_dump($_SESSION['status_venta']);
    //echo var_dump($_SESSION['ultimo_id']);

    $obj = new BD_PDO("usuarios");


    if (isset($_POST['btninsertardetalle'])) 
    {
        //if(!isset($_SESSION['status_venta']))
        //{
         if($_SESSION['status_renta']=='Cerrada')
         {
            $obj->Ejecutar_Instruccion("insert into rentas(fecharenta,idusuario,idcliente) values(Now(),'".$_POST['txtusuario']."','".$_POST['txtcliente']."')");
            $_SESSION['ultimo_id'] = $obj->Ultimo_id();
            $_SESSION['status_renta']='Abierta';
         }

    $result = $obj->Ejecutar_Instruccion("insert into detalle_renta(idrenta,idauto) values('".$_SESSION['ultimo_id']."','".$_POST['txtidauto']."')");
        /*}
        else
        {
            $_SESSION['ultimo_id']=0;
        }*/
    }
    elseif (isset($_POST['btncerrarrenta'])) 
    {
        session_destroy();
        // header("Location: reporte_ticket.php");
        unset($_SESSION['status_renta']);
        $_SESSION['status_renta']='Cerrada';
    }

    // $datos = $obj->Ejecutar_Instruccion("Select IdDetalleVenta, IdVenta,NombreProducto,IdProducto, Cantidad,Precio,( Cantidad*Precio ) as Subtotal  from tbldetalleventa inner join tblproducto on tbldetalleventa.IdProducto=tblproducto.id_Producto where IdVenta='".@$_SESSION['ultimo_id']."'");


    //$ticket = $obj->Ejecutar_Instruccion("CALL ticket('82');");
    //echo var_dump($ticket);

 ?>