haversine.php

<?php 
/**
* Based on Moodle 3.x Developer's Guide book example 
*/
class Haversine
{
    public $radius = 6378100; // Meters
    
    public function get_distance($x1, $y1, $x2, $y2)
    {
        $lon_arc = deg2rad(($x1 - $x2));
        $lat_arc = deg2rad(($y1 - $y2));
        $lonh = sin($lon_arc * 0.5);
        $lonh *= $lonh;
        $lath = sin($lat_arc * 0.5);
        $lath *= $lath;
        $tmp = cos(deg2rad($y1)) * cos(deg2rad($y2));
        $distance = 2 * $this->radius * asin(sqrt($lath + $tmp * $lonh));
        return $distance;
    }
}