결국 쓰게 되는군.

include "geotrans.php";

$naverx = "306151";
$navery = "556443";

$geo = new GeoTrans;
$geo->conv($naverx,$navery,$x,$y);

echo("======input naver postion=======n");
echo("naver x: ".$naverx);
echo("n");
echo("naver y: ".$navery);
echo("n");
echo("======convert naver to google=======n");
echo("google lng: ".$x);
echo("n");
echo("google lat: ".$y);
echo("n");

$geo = new GeoTrans("geo","katec");
$geo->conv($x,$y,$final_naverx,$final_navery);
echo("======convert google to naver=======n");
echo("final naver x: ".$final_naverx);
echo("n");
echo("final naver y: ".$final_navery);
echo("n");