Function to create a circle with a radius of X km, centered on a lat lon

envelop_point.php

<?php

/**
 * Envelop a coordinate in a circle.
 *
 * @param string $lat
 *  Latitude.
 * @param string $lon
 *  Longitude.
 * @param float|int $radius
 *  Radius in km.
 * @param int $steps
 *  Optional. The number of vertexes in the circle. Defaults to 8.
 * @param bool $close
 *  Optional. Whether to close the circle. Defaults to FALSE.
 * @return array
 */
function envelop_point($lat, $lon, $radius, $steps=8, $close=FALSE) {
  $R = 6371; // The radius of earth

  $delta_lon = rad2deg($radius/$R/cos(deg2rad($lat)));
  $delta_lat = rad2deg($radius/$R);

  $pts = array();
  $step = pi() / ($steps / 2);
  if ($close) {
    $steps++;
  }
  
  for ($i = 0; $i < $steps; $i++) {
    $rad = $step * $i;
    $pts[] = array(
      $lat + $delta_lat * cos($rad),
      $lon + $delta_lon * sin($rad),
    );
  }
  return $pts;
}

$pts = array();
foreach (envelop_point(55.6, 13.0, 3) as $c) {
  $pts[] = "{$c[0]},{$c[1]}";
}
$coords = join('|', $pts);
shell_exec("open 'http://maps.google.com/maps/api/staticmap?path=color:0xff0000ff|weight:5|{$coords}&size=512x512&sensor=false'");