Security Token?

Hi, i am trying to come up with a security token system for all my form, normal approach would be setting a token and put it in to the session whenerever the user request a new page.

$_SESSION['token'] = random string here.

The problem is, the string would be overwritten everytime when a user request a new page.
But it is common now for people to open up a few tab for a website.
And it would cause the token to expire, and cause the last few tabs you open to fail the security token check.

Is there a better soultion?

What type of form do you have? Is it a contact form? And are you just trying to prevent spam? I suggest you read this article if its a contact form.

http://www.phpbuilder.com/columns/ian_gilfillan20060412.php3

Actually it is more of preventing spam, more like forcing the user to use my form to submit the data.