Grow a lat lon geobox based on a given radius in kilometres and a centre point

gistfile1.php

function point_distance($coords, $center)
{ 
    $radius = 6378100; // radius of earth in meters
    $latDist = $center[0] - $coords[0];
    $lngDist = $center[1] - $coords[1];
    $latDistRad = deg2rad($latDist);
    $lngDistRad = deg2rad($lngDist);
    $sinLatD = sin($latDistRad);
    $sinLngD = sin($lngDistRad);
    $cosLat1 = cos(deg2rad($center[0]));
    $cosLat2 = cos(deg2rad($coords[0]));
    $a = ($sinLatD/2)*($sinLatD/2) + $cosLat1*$cosLat2*($sinLngD/2)*($sinLngD/2);
    if($a<0) $a = -1*$a;
    $c = 2*atan2(sqrt($a), sqrt(1-$a));
    $distance = $radius*$c;
    return $distance;
}

function grow_box($radius, $coords, $center)
{
    $center = explode(',', $center);

    $ne_distance = (point_distance(array($coords['latn'], $coords['lone']), $center));
    $ne_ratio = ($radius*1000) / $ne_distance;

    $coords['latn'] = $coords['latn'] - (($center[0] - $coords['latn']) * $ne_ratio);
    $coords['lone'] = $coords['lone'] - (($center[1] - $coords['lone']) * $ne_ratio);

    $sw_distance = (point_distance(array($coords['lats'], $coords['lonw']), $center));
    $sw_ratio = ($radius*1000) / $ne_distance;

    $coords['lats'] = $coords['lats'] - (($center[0] - $coords['lats']) * $ne_ratio);
    $coords['lonw'] = $coords['lonw'] - (($center[1] - $coords['lonw']) * $ne_ratio);

    return $coords;
}

$coords = array(
  'latn' => 51.6679,
  'lone' => 0.137323,
  'lats' => 51.3856,
  'lonw' => -0.347998,
);

$center = '51.5149,-0.108381';

$new_box = grow_box(10, $coords, $center);