model_propel.php

<?php

// lib/model/ItemQuery.php

class ItemQuery extends BaseItemQuery
{
  /**
   * @param  array            $values
   * @param  integer          $limit
   * @return PropelCollection
   */
  public function search($values, $limit = 10)
  {
    $lat = floatval($values['latitude']);
    $lng = floatval($values['longitude']);

    // replace 6371 with 3959 if you want miles instead of kilometres
    return $this->create()
      ->withColumn('(6371 * acos(cos(radians(' . $lat . ')) * cos(radians(latitude)) * cos(radians(longitude) - radians(' . $lng . ')) + sin(radians(' . $lat . ')) * sin(radians(latitude))))', 'distance')
      ->orderBy('distance')
      ->where('distance <= ?', $values['radius'])
      ->limit($limit)
      ->find();
  }
}