Серафим Иванов

if (isset($_GET['utm_source'])){
   $url_add = '?utm_source='.$_GET['utm_source'];
}

else {
  $url_add = false;
}

<a href="http://2example.com/order<?= $url_add?>">Купить</a>

RewriteCond %{HTTP_COOKIE} (?:^|;\s*)city=([^;]+)
RewriteCond "%1  moskva=url1  omsk=url2  orsk=url3" "^(\S+) .*?\1=(\S+)"
RewriteRule ^  /%2 [R=301,L]