haversini formula - PHP working

haversini.php

<?php

function getDistance($latitude1, $longitude1, $latitude2, $longitude2) {
    
    $earth_radius = 6371;

    $distanceLat = deg2rad($latitude2 - $latitude1);
    $distanceLon = deg2rad($longitude2 - $longitude1);

    //part1 of the formula
    $part1 = sin($distanceLat/2) * sin($distanceLat/2) + cos(deg2rad($latitude1)) * cos(deg2rad($latitude2)) * sin($distanceLon/2) * sin($distanceLon/2);
    
    //part2 of the formula
    $part2 = 2 * asin(sqrt($part1));
    
    //part3 of the formula
    $result = $earth_radius * $part2;
    
    //return value
    return $result;
    
}