here's what i would do....
To get this:
tour1.php?from=yahoo
tour2.php?from=yahoo
tour3.php?from=yahoo
on the forms do this:
<a href='tour2.php?from=$from'>Next</a>
<a href='tour3.php?from=$from'>Next</a>
<a href='signup.php?from=$from'>Signup</a>
**EDIT**
PHP Code:
<a href='tour2.php?from=$from'>Next</a>
<a href='tour3.php?from=$from'>Next</a>
<a href='signup.php?from=$from'>Signup</a>
**/EDIT**
Or at least that's what I would do...you could try playing with cookies, but I wouldn't recommend it (but you could do it if you were trying to hide the "from=yahoo" part.
Let me know if that works for you!