Haversine formula

hf.php

/**
 * Calculates the geodesic distance between two points specified by radian latitude/longitude using the
 * Haversine formula (hf)
 */
function hf($lat1, $lng1, $lat2, $lng2)
{
    $R = 6371e3;

    $lng1 = deg2rad($lng1);
    $lat1 = deg2rad($lat1);
    $lng2 = deg2rad($lng2);
    $lat2 = deg2rad($lat2);

    $deltaLat = $lng2 - $lng1;
    $deltaLng = $lat2 - $lat1;

    $a = (sin($deltaLat / 2) ** 2) + (cos($lng1) * cos($lng2) * (sin($deltaLng / 2) ** 2));
    $c = 2 * asin(min(1, sqrt($a)));
    $d = $R * $c;

    return $d;
}