Measuring distance from two Geo-Points in PHP

geo-distance.php

function distance($lat1, $long1, $lat2, $long2) {
  	$R = 6371; // Earth Radius in Km
	$dLat = deg2rad($lat2-$lat1);
	$dLong = deg2rad($long2-$long1);
	$lat1 = deg2rad($lat1);
	$lat2 = deg2rad($lat2);

	$a = sin($dLat/2)*sin($dLat/2) + sin($dLong/2)*sin($dLong/2)*cos($lat1)*cos($lat2);
	$c = 2 * atan2(sqrt($a),sqrt(1-$a));
	$d = $R * $c;
	// Return with the distance
	return $d; 
}