隐藏网址中的值

I need to pass the value 'key' to securitycode.php file. This is my current code.

http://mydomain.com/securitycode.php?key=12345678

But I don't want the key value to be accessible through url (it is visible through browsers' 'view page source' functionality, which means spambots can easily access it, too). Since it's for security, that would make it meaningleass. (it's for captcha.)

How do I pass the value without using url?

I can't use the post method with form because it's already in another form. I think I can save the value in session but I would like to know if there is any other simpler way not using session.

If you're doing captcha, the key should be stored in a session variable and not shared with the browser at all.

session_start();
// ...
$_SESSION['key'] = "123456789";