Measuring the distance between two coordinates in PHP

<?php

/**
 * Calculate the Haversine distance between two points on the Earth's surface
 *
 * @param float $lat1 The latitude of the first point
 * @param float $lon1 The longitude of the first point
 * @param float $lat2 The latitude of the second point
 * @param float $lon2 The longitude of the second point
 * @return float The distance between the two points, in kilometers
 */
function haversine_distance($lat1, $lon1, $lat2, $lon2)
{
  $radius = 6371; // Earth's radius in kilometers

  // Calculate the differences in latitude and longitude
  $delta_lat = $lat2 - $lat1;
  $delta_lon = $lon2 - $lon1;

  // Calculate the central angles between the two points
  $alpha = $delta_lat / 2;
  $beta = $delta_lon / 2;

  // Use the Haversine formula to calculate the distance
  $a = sin(deg2rad($alpha)) * sin(deg2rad($alpha)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * sin(deg2rad($beta)) * sin(deg2rad($beta));
  $c = asin(min(1, sqrt($a)));
  $distance = 2 * $radius * $c;

  // Round the distance to four decimal places
  $distance = round($distance, 4);

  return $distance;
}

// Sample usage
$lat1 = 40.712776;
$lon1 = -74.005974;
$lat2 = 51.507351;
$lon2 = -0.127758;

$distance = haversine_distance($lat1, $lon1, $lat2, $lon2);
echo "The distance between ($lat1, $lon1) and ($lat2, $lon2) is $distance km.";