Calculate distance based on longitude and latitude in PHP

This is a very useful distance calculation function that uses latitude and longitude to calculate the distance from point A to point B. This function can return distance in three unit types: miles, kilometers, and nautical miles.

function distance($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);

if ($unit == "K") {
return ($miles * 1.609344);
} else if ($unit == "N") {
return ($miles * 0.8684);
} else {
return $miles;
}
}

//Usage:
echo distance(32.9697, -96.80322, 29.46786, -98.53506, "k")." kilometers";

PHP