计算地球上2个经纬度之间的距离

采用半正矢公式（haversine formula）：

/**
 * Calculates the great-circle distance between two points, with
 * the Haversine formula.
 * @param float $latitudeFrom Latitude of start point in [deg decimal]
 * @param float $longitudeFrom Longitude of start point in [deg decimal]
 * @param float $latitudeTo Latitude of target point in [deg decimal]
 * @param float $longitudeTo Longitude of target point in [deg decimal]
 * @param float $earthRadius Mean earth radius in [m]
 * @return float Distance between points in [m] (same as earthRadius)
 */
function haversineGreatCircleDistance($latitudeFrom, $longitudeFrom, $latitudeTo, $longitudeTo, $earthRadius = 6371000){
    // convert from degrees to radians
    $latFrom = deg2rad($latitudeFrom);
    $lonFrom = deg2rad($longitudeFrom);
    $latTo = deg2rad($latitudeTo);
    $lonTo = deg2rad($longitudeTo);

    $latDelta = $latTo - $latFrom;
    $lonDelta = $lonTo - $lonFrom;

    $angle = 2 * asin(sqrt(pow(sin($latDelta / 2), 2) +
        cos($latFrom) * cos($latTo) * pow(sin($lonDelta / 2), 2)));
    return $angle * $earthRadius;
}

返回的距离单位和你传递的$earthRadius的单位一致。如默认是6371000米，返回的单位也是米。

MySQL：

6371000 * 2 * ASIN( SQRT( POW( SIN((lat1*PI()/180-lat2*PI()/180)/2), 2) + COS(lat1*PI()/180) * COS(lat2*PI()/180) * POW( SIN((lng1*PI()/180 - lng2*PI()/180)/2), 2) ) )

采用文森特公式（Vincenty formula）：

/**
 * Calculates the great-circle distance between two points, with
 * the Vincenty formula.
 * @param float $latitudeFrom Latitude of start point in [deg decimal]
 * @param float $longitudeFrom Longitude of start point in [deg decimal]
 * @param float $latitudeTo Latitude of target point in [deg decimal]
 * @param float $longitudeTo Longitude of target point in [deg decimal]
 * @param float $earthRadius Mean earth radius in [m]
 * @return float Distance between points in [m] (same as earthRadius)
 */
public static function vincentyGreatCircleDistance($latitudeFrom, $longitudeFrom, $latitudeTo, $longitudeTo, $earthRadius = 6371000){
    // convert from degrees to radians
    $latFrom = deg2rad($latitudeFrom);
    $lonFrom = deg2rad($longitudeFrom);
    $latTo = deg2rad($latitudeTo);
    $lonTo = deg2rad($longitudeTo);

    $lonDelta = $lonTo - $lonFrom;
    $a = pow(cos($latTo) * sin($lonDelta), 2) +
        pow(cos($latFrom) * sin($latTo) - sin($latFrom) * cos($latTo) * cos($lonDelta), 2);
    $b = sin($latFrom) * sin($latTo) + cos($latFrom) * cos($latTo) * cos($lonDelta);

    $angle = atan2(sqrt($a), $b);
    return $angle * $earthRadius;
}

采用文森特公式的精度很高，能达到0.5毫米，但是速度很慢；半正矢公式速度比文森特快，但精度没有文森特高。