PHP根据两点间的经纬度计算距离

/**
*  说明: 根据两点间的经纬度计算距离
*  @param float $lat 纬度值
*  @param float $lng 经度值
*/
 function getDistance($lat1, $lng1, $lat2, $lng2) {
     $earthRadius = 6367000; 

     $lat1 = ($lat1 * pi() ) / 180;
     $lng1 = ($lng1 * pi() ) / 180;

     $lat2 = ($lat2 * pi() ) / 180;
     $lng2 = ($lng2 * pi() ) / 180;

     $calcLongitude = $lng2 - $lng1;
     $calcLatitude = $lat2 - $lat1;
     $stepOne = pow(sin($calcLatitude / 2), 2) + cos($lat1) * cos($lat2) * pow(sin($calcLongitude / 2), 2);  
	 $stepTwo = 2 * asin(min(1, sqrt($stepOne)));
     $calculatedDistance = $earthRadius * $stepTwo;

     return round($calculatedDistance);
 }

$a = getDistance('39.916155', '116.396885', '39.910329', '116.396443');
echo($a);