The site is protected by password in this moment, because it's under development.
Anywhow I worked all the night to solve this issue.
I found it a couple of hours ago. The fault was mine.
I was using a created PayPal button, using the code that PayPal assigns to the buttons.
The fact is that, with their buttons, I cannot assign a value to that field from my website.
When I use their button , I have to fix the parameters in their form, and I'm not allowed to use anything else.
The solution was and is: to avoid their buttons.
Just creating the form as I published in my first message here, I can use all the parameters I want, and these are created by several functions in the website itself.
I didn't know that, that's why I was creating confused things.
I'm sorry for my mistakes.
A "mistery" stays the return page.
Indeed in this case, if I write:
<input type="hidden" name="return" valuer="http://www.mysite.com/return_page.php" />
PayPal appends in GET (I wanted in POST but I have not clue how to do it) some flag,
while if I want to have added some my flags to the one of PayPal, like this example:
<input type="hidden" name="return" value="http://www.mysite.com/return_page.php?sid=$sid&type=$type" /> where $sid and $type are generated by my PHP script, what I get back is something like:
and PayPal doesn't add anything like it should. Indeed I expected to have back:
but it doesn't :-(