PHP Haversine - Calculate the distance between two points considering the curvature of earth (useful for long distances)

haversine.php

<?php
/*
 * PHP Haversine - Calculate the distance between two points considering the curvature of earth
 * Checkmate, flat-earthers!
 * 
 * The result will be given in km. if you want meters, set $earth_radius to 6371000.
 */
function getDistance($latitude1, $longitude1, $latitude2, $longitude2) {
    $earth_radius = 6371; // this is in km so the result will be in km as well

    $dLat = deg2rad($latitude2 - $latitude1);
    $dLon = deg2rad($longitude2 - $longitude1);

    $a = sin($dLat/2) * sin($dLat/2) + cos(deg2rad($latitude1)) * cos(deg2rad($latitude2)) * sin($dLon/2) * sin($dLon/2);
    $c = 2 * asin(sqrt($a));
    $d = $earth_radius * $c;

    return $d;
}

$p1lat = 47.2173788;
$p1lon = 6.7867624;

$p2lat = 51.2073988;
$p2lon = 8.7967620;

echo getDistance($p1lat, $p1lon, $p2lat, $p2lon);