Setting a cookie upon specific URL visit

I’ve used this and it works:

 add_action('init', 'optin_cookie', 1);

 function optin_cookie(){

   $currentURL = "https://" . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];

   if ( $currentURL == 'https://25dni.si/delovanje-uma/' ) :

    setcookie( 'opt_in', has_opt_in, time()+31556926, "https://wordpress.stackexchange.com/");

   elseif ( $currentURL != 'https://25dni.si/' ) :

     if ( ! isset( $_COOKIE['opt_in'] ) ) :

     endif;

    endif;

 }

I’m sure there is a much more elegant way to do it, but this got it working for me …