Setting a Cookie if Hash is Valid in URL

what I'm doing is passing a hash in the URL from page1.php to page2.php which has been working fine so far but instead of setting $valid to TRUE I want to set a cookie.

Currently I have the code working like this:

if ($hash === md5($key.$test_time)) $valid = TRUE;

To set the cookie instead I tried this:

if ($hash === md5($key.$test_time))
//set "Cookie" for 24 hours
$cookie_value = "Cookie";
setcookie("Placed", $cookie_value, time()+86400);

I am getting these errors:

Notice: Undefined variable: cookie_value in /Dev/sites/files/page2.php on line 19

and

Warning: Cannot modify header information - headers already sent by (output started at /Dev/sites/files/page2.php:19) in /Dev/sites/files/page2.php on line 19

How do I fix this so I can set a cookie instead of a value?

via Chebli Mohamed