Lat/Lon Distance Calculator

geoDistance.php

<?php
/**
 *  Calculate the distance between two points on a sphere
 *
 *  Implements the Haversine formula: http://en.wikipedia.org/wiki/Haversine_formula
 *
 *  More advanced calculation (good for antipodal points) can be used by using
 *      FALSE for $boolUseHaversine. This will use another solution to the Great-circle
 *      ditance problem (http://en.wikipedia.org/wiki/Great-circle_distance), implementing
 *      the Vincenty formula (http://en.wikipedia.org/wiki/Vincenty%27s_formulae#Nearly_antipodal_points).
 *      This matches the calculation from Google's Geometry Library's computeDistanceBetween() function,
 *      assuming the Mercator projection value of Earth's radius of 6378137.0 meters
 *
 *  $lat1 (string) decimal latitude for point 1
 *  $lng1 (string) decimal longitude for point 1
 *  $lat2 (string) decimal latitude for point 2
 *  $lng2 (string) decimal longitude for point 2
 *  $unit (string) OPTIONAL unit of measure, either M (miles, the default), K (kilometers), or N (nautical miles)
 *  $boolUseHaversine (boolean) OPTIONAL formula to use for the calculation. Haversine is good for shorter distances (<10,000 km),
 *        see above for more information on Haversine
 *
 *  @return Returns a decimal distance in the $unit of choice
 *
 */
function geoDistance($lat1, $lng1, $lat2, $lng2, $unit = "M", $boolUseHaversine = true)
{
    if ($boolUseHaversine === true) {
        $theta = $lng1 - $lng2;
        $dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
        $dist = acos($dist);
        $dist = rad2deg($dist);
        $totalDist = $dist * 60 * 1.1515;
        $unit = strtoupper($unit);

        if ($unit == "K") {
            return ($totalDist * 1.609344);
        } elseif ($unit == "N") {
            return ($totalDist * 0.8684);
        } else {
            return $totalDist;
        }
    } else {
        // Vincenty formula implementation from martinstoeckli on Stack Overflow: http://stackoverflow.com/a/10054282/1026279

        $earthRadius = 6378137.0; // in meters
        
        // convert from degrees to radians
        $latFrom = deg2rad($lat1);
        $lonFrom = deg2rad($lng1);
        $latTo = deg2rad($lat2);
        $lonTo = deg2rad($lng2);

        $lonDelta = $lonTo - $lonFrom;
        $a = pow(cos($latTo) * sin($lonDelta), 2) +
        pow(cos($latFrom) * sin($latTo) - sin($latFrom) * cos($latTo) * cos($lonDelta), 2);
        $b = sin($latFrom) * sin($latTo) + cos($latFrom) * cos($latTo) * cos($lonDelta);

        $angle = atan2(sqrt($a), $b);
        $totalDist = $angle * $earthRadius;

        return $totalDist / 1000;
    }
}