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    <?php

    namespace WPGMZA;

    class Distance
    {
        const UNITS_KM      = "km";
        const UNITS_MI      = "mi";

        const MILES_PER_KILOMETER = 0.621371;
        const KILOMETERS_PER_MILE = 1.60934;

        public static function between($a, $b)
        {
            if(!($a instanceof LatLng))
                throw new \Exception('First argument is not an instance of WPGMZA\LatLng');

            if(!($b instanceof LatLng))
                throw new \Exception('Second argument is not an instance of WPGMZA\LatLng');

            $lat1 = $a->lat;
            $lon1 = $a->lng;
            $lat2 = $b->lat;
            $lon2 = $b->lng;

            $dLat = deg2rad($lat2 - $lat1);
            $dLon = deg2rad($lon2 - $lon1);

            $a =
                sin($dLat / 2) * sin($dLat / 2) +
                cos(deg2rad($lat1)) * cos(deg2rad($lat2)) *
                sin($dLon / 2) * sin($dLon / 2);

            $c = 2 * atan2(sqrt($a), sqrt(1 - $a));
            $d = 6371 * $c;

            return $d;
        }
    }