distance between Geocodes calculation:

function getDistanceBetweenPointsNew($latitude1, $longitude1, $latitude2, $longitude2, $unit = 'Mi') {
$theta = $longitude1 - $longitude2;
$distance = (sin(deg2rad($latitude1)) * sin(deg2rad($latitude2))) + (cos(deg2rad($latitude1)) * cos(deg2rad($latitude2)) *
cos(deg2rad($theta)));
$distance = acos($distance);
$distance = rad2deg($distance);
$distance = $distance * 60 * 1.1515;
switch($unit) {
case ‘Mi’: break;
case ‘Km’ : $distance = $distance * 1.609344;
}return (round($distance,2));
}

Ref: http://marketingtechblog.com/2007/09/15/calculate-distance/