PHP cookie和时间

I'm am experimenting with cookies for the first time and I don't think I'm understanding this or my code is doing something loopy. Someone mind helping me get back on the right path?

I keep re reading the php.net help but i think my mind is mush now :/

$currentTime = strtotime("now");
$popup_exp = strtotime("+1 hour");

if (!isset($_COOKIE['popup_timer'])) : //does cookie exists? if not, make it
    setcookie("popup_timer", $currentTime);
endif;
if( ($popup_exp > $_COOKIE['popup_timer']) ): 
    //show my popup
endif;

Not getting your code quiet well, but as you commented

if(!isset($_COOKIE['popup_timer'])) {
   //Show popup
   setcookie("popup_timer", '', time()+3600);
}

The above will throw a pop up only if the $_COOKIE is not set, once it throws the pop up, a cookie will be set with an expiration set to an hour.

You should compare your current time with the time stored in the cookie, like this

$currentTime = strtotime('now');

if (!isset($_COOKIE['popup_timer'])) {
    setcookie('popup_timer', $currentTime);
} else {
    if ($currentTime > $_COOKIE['popup_timer'] + 60 * 60) {
        // If an hour has passed since cookie creation
        // show your popup
    }
}