PHP función para saber si navegas por http o https

function protocolo_web()
{
    $protocol =  "http://";
    if (
        //straight
        isset($_SERVER['HTTPS']) && in_array($_SERVER['HTTPS'], ['on', 1])
        ||
        //proxy forwarding
        isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https'
    ) {
        $protocol = 'https://';
    }

    //$domainName = $_SERVER['HTTP_HOST'];
    //return $protocol . $domainName;
	return $protocol ;
}

Modo de uso

$protocol = protocolo_web();

Referencia

https://stackoverflow.com/questions/4503135/php-get-site-url-protocol-http-vs-https