如何从点计算半径纬度 - 经度

I would like to know how do I add 50 kilometers to a map point (latitude / longitude) (radius in 50 kilometers).

For example: Think I have latitude -26.462049, longitude -49.059448. I want to add 100 kilometers up, down, and on the sides. I want to do a square and get the new points.

How do I do that? And how to get all value from database.

$radius = 100  //range
$lat_range = $radius/69.172;  
$lon_range = abs($radius/(cos($latitude) * 69.172));  
$min_lat = number_format($latitude - $lat_range, "4", ".", "");  
$max_lat = number_format($latitude + $lat_range, "4", ".", "");  
$min_lon = number_format($longitude - $lon_range, "4", ".", "");  
$max_lon = number_format($longitude + $lon_range, "4", ".", "");  

//Query like this ()

 $sqlstr = mysql_query("SELECT * FROM `tablename`   WHERE (`latitude` BETWEEN '".$min_lat."' AND '".$max_lat."') AND  (`longitude` BETWEEN '".$min_lon."' AND '".$max_lon."') "); 

        function calculateDistance($latitude1, $longitude1, $latitude2, $longitude2) 
        {
            $theta = $longitude1 - $longitude2;
            $miles = (sin(deg2rad($latitude1)) * sin(deg2rad($latitude2))) + (cos(deg2rad($latitude1)) * cos(deg2rad($latitude2)) * cos(deg2rad($theta)));
            $miles = acos($miles);
            $miles = rad2deg($miles);
            $miles = $miles * 60 * 1.1515;
            return $miles; 
        }
        $point1_lat = '-26.462049';
        $point1_lng = ' -49.059448.';

        $point2_lat = currentlatitute;
        $point2_lng = currentlongitude;

        $miles = calculateDistance($point1_lat, $point1_lng, $point2_lat,$point2_lng);
        $kms = $miles * 1.609344;

        echo $kms;