Make this code only execute once per visitor, help appreciated


New member
Hi I am using this code in my index.php to send myself android notification about what sites are referring visitors to my website.
Issue I have is that I usually get 2 even 3 notification per visitor sometimes.
Is there any way to edit this code so that it only send 1 notification per referrer?

if (!empty($_SERVER['HTTP_REFERER'])) {               
    $refer = $_SERVER['HTTP_REFERER'];

$word1 = "google.";

if (!empty($refer) and !strpos($refer, $word1)) {
        // Target URL
    $url = "{$refer}&u={$refer}";
        // Fetching headers, aka visit/ping $url above
    $headers = get_headers($url);