依据经纬度计算距离的公式、百度坐标转换成GPS坐标(PHP版)

<?php
//百度坐标转换成GPS坐标
$lnglat = '121.437518,31.224665';
function FromBaiduToGpsXY($lnglat){
	// 经度,纬度
	$lnglat = explode(',',$lnglat);
	list($x,$y) = $lnglat;
	$Baidu_Server = "http://api.map.baidu.com/ag/coord/convert?from=0&to=4&x={$x}&y={$y}";
	$result = @file_get_contents($Baidu_Server);
	$json = json_decode($result);
	if($json->error == 0)
	{
		$bx = base64_decode($json->x);	
		$by = base64_decode($json->y);
		$GPS_x = 2 * $x - $bx;
		$GPS_y = 2 * $y - $by;
		return $GPS_x.','.$GPS_y;//经度,纬度
	}
        else
          return  $lnglat;
}
/**********************************************/
function fn_rad($d)
{
	return $d * pi() / 180.0;
}
// 2点间算法
function P2PDistance($latlng1,$latlng2)
{
	// 纬度1,经度1 ~ 纬度2,经度2
	$latlng1 = explode(',',$latlng1);
	$latlng2 = explode(',',$latlng2);
	list($lat1,$lng1) = $latlng1;
	list($lat2,$lng2) = $latlng2;
	$EARTH_RADIUS = 6378.137;
	$radLat1 = fn_rad($lat1);
	$radLat2 = fn_rad($lat2);
	$a = $radLat1 - $radLat2;
    $b = fn_rad($lng1) - fn_rad($lng2);
	$s = 2 * asin(sqrt(pow(sin($a/2),2) + cos($radLat1)*cos($radLat2)*pow(sin($b/2),2)));
	$s = $s * $EARTH_RADIUS;
	$s = round($s * 10000) / 10000;
	return number_format($s,2);
}
echo '百度坐标: ',$lnglat,'<br><br>','转换后GPS坐标: ',FromBaiduToGpsXY($lnglat),'<br><br>';
echo '转换前距离: ',P2PDistance('31.224286666667,121.420675','31.224665,121.437518'),'<br/>';
echo '转换后距离: ',P2PDistance('31.224286666667,121.420675','31.220157068379,121.42647022694');
?>