you can find a similar question here PHP Post Request inside a POST Request but this is not working in my context.
I have a form (reservation form for a tour website) an
Well, I suppose that if you are talking about keeping data in your own website between pages you need to use PHP's session functions
To start the session just do session_start();
and to add session vars just use the superglobal array $_SESSION['myvar'] = $value;
you can then read them through the same means print_r($_SESSION[]);
However if you are talking about sensing data with the paypal API I highly recommend looking at their developer API manual.
Hope that helps, RayQuang