use http build query

function sendInviteMail($to, $secret, $lang, ?string $referrer = null, ?string $shopLocation = null)

{

    $encoded_email = urlencode($to);

    $domain = getenv("DOMAIN");

    $signupURL = "https://$domain/signup/";

    if ($lang != "en") {

        $signupURL .= "$lang/";

    }

    $signupURL .= "register?authmail=$encoded_email&authsecret=$secret";

    $signupURL .= "register?";

    $query = [

        "authmail"=> $to,

        "authsecret"=>$secret

        ];

    if ($referrer != null) {

        $signupURL .= "&ref=$referrer";

        $query["ref"] = $referrer;

    }

    if ($shopLocation) {

        $signupURL .= "&shop=$shopLocation";

        $query["shop"] = $shopLocation;

    }

    $SENDGRID_API_KEY = getenv("SENDGRID_API_KEY");

    $signupURL = urlencode($signupURL);

    $signupURL = $signupURL . http_build_query($query);

    if (!empty($SENDGRID_API_KEY)) {

        return sendInviteMailWithSendGrid($to, $signupURL);

    }