class POI {
    private $latitude;
    private $longitude;

    public function __construct($latitude, $longitude) {
        $this->latitude = deg2rad($latitude);
        $this->longitude = deg2rad($longitude);
    }

    public function getLatitude() {
        return $this->latitude;
    }

    public function getLongitude() {
        return $this->longitude;
    }

    public function getDistanceInMetersTo(POI $other) {
        $radiusOfEarth = 6371; // Earth's radius in kilometers.

        $diffLatitude = $other->getLatitude() - $this->latitude;
        $diffLongitude = $other->getLongitude() - $this->longitude;

        $a = sin($diffLatitude / 2) ** 2 +
             cos($this->latitude) *
             cos($other->getLatitude()) *
             sin($diffLongitude / 2) ** 2;

        $c = 2 * asin(sqrt($a));
        $distance = $radiusOfEarth * $c;

        return $distance;
    }
}