$ratio = 1;
if ($curwidth > $maxwidth) $ratio = $maxwidth / $curwidth;
if ($curheight > $maxheight) {
	$hratio = $maxheight / $curheight;
	if ($hratio < $ratio) $ratio = $hratio; 
}

$maxwidth = $curwidth * $ratio;
$maxheight = $curheight * $ratio;