How to compute distance between 2 GPS points?

In one of my projects I was looking for simple way how to compute direct distance between 2 arbitrary airports in the world. Simply, I was looking for a way to measure distance between 2 GPS points. Naturally there exist several ways how to compute such thing. After research I found that many people use algorithms derived from Great circle distance method [wiki].

Here is a PHP code:

    /**
     * Great Circle distance algorithm
     *
     * Notes:
     * - South and West locations are negative, North and East are positive.
     * - Automatically transfer degrees to radians
     * - https://en.wikipedia.org/wiki/Great-circle_distance
     *
     * @param lat1 - latitude of first point on sphere
     * @param lon1 - longitude of first point on sphere
     * @param lat2 - latitude of second point on sphere
     * @param lon2 - longitude of second point on sphere
     * @param unit - units for output (miles, kilometers, nautical miles)
     * @return - distance in desired distance units
     *
     */
    function GCdistance($lat1, $lon1, $lat2, $lon2, $unit) {
        $theta = $lon1 - $lon2;
        $dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) +  cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
        $dist = acos($dist);
        $dist = rad2deg($dist);
        $miles = $dist * 60 * 1.1515;
        $unit = strtoupper($unit);

        switch ($unit) {
            case 'MI':
                return $miles;
                break;
            case 'KM':
                return ($miles * 1.609344);
                break;
            case 'NM':
                return ($miles * 0.8684);
                break;
        }
    }

There are 2 simple corrections how to simplifie this algorithm more. First by padding all the sub-results into one big equation. And second, by ensuring the the input units will be always the same (using numbers instead of Strings) strtoupper() function can be removed.