How to set cookie value properly with $_SERVER[‘REQUEST_URI’]?

I have a Google AdWords campaign, so when users click on the ad it links them to a certain page. The page URL has a specific Google Ad string, so using it, I want to change some colors on all pages, not only one.

I’m trying to use $_SERVER['REQUEST_URI'] and setcookies() function. Let’s say that the Google Ad string is adword. So when the ad was clicked it linked to https://example.com/adword/ page and I get this slug string and set to cookies a value:

$cookie_val = '';
if (strpos($_SERVER['REQUEST_URI'], "adword") && !isset($_COOKIE['ad_cookies'])){
    setcookie('ad_cookies', 'myvalue', time()+(3600*6));  /* expire in 6 hours */
    $cookie_val = $_COOKIE['ad_cookies'];
}

I though then I can use this cookie value everywhere, for example by changing class name of a div:

<div class="page <?php echo $cookie_val ?>">

But it only works on one page https://example.com/adword/.

So my question is, how can I use this cookie value on every page when a user clicked the ad? Thanks in advance!