$xtile = (($_GET['lon'] + 180) / 360) * pow(2, $zoom);
  $ytile = (1 - log(tan(deg2rad($_GET['lat'])) + 1 / cos(deg2rad($_GET['lat']))) / pi()) /2 * pow(2, $zoom);
  $rechts = $xtile + ($xtile/2);
  $links = $xtile - ($xtile/2);
  $oben = $ytile - ($ytile/2);
  $unten = $ytile + ($ytile/2);
  $n = pow(2, $zoom);
  $lat_r = $rechts / $n * 360.0 - 180.0;
  $lon_o = rad2deg(atan(sinh(pi() * (1 - 2 * $oben / $n))));
  $lat_l = $links / $n * 360.0 - 180.0;
  $lon_u = rad2deg(atan(sinh(pi() * (1 - 2 * $unten / $n))));