Thanks both for the feedback.
Somehow I thought it would be as easy as one line of code (possibly simply
another header with POST data instead of the data in the URL).
Because I'm really not that good using php (first steps) and certainly not
cookies (know nothing about it) and this is only for a "fun after work" type
thing for a local youth group (project due soon) I'm not going to bother
implementing more complex things (although I do get inspiration from these
posts ;-)) but I have to refrain myself and stick to the more needed things
that still need implementing.