Answer by miken32 for How to detect users setting do-not-track

It's sent as an HTTP header:

function dnt_enabled()
{
   return (isset($_SERVER['HTTP_DNT']) && $_SERVER['HTTP_DNT'] == 1);
}

if dnt_enabled() {
    // do stuff...
}

Or, if you're using PHP 7:

function dnt_enabled(): bool
{
   return (bool)$_SERVER['HTTP_DNT'] ?? false;
}