Código PHP:
$markers = new Application_Model_DbTable_Markers();
$select = $markers->select();
$select->from($markers, array( //podría poner 'markers'(tabla)
'address','name','lat',
'distance' => '6371*acos(cos(radians($center_lat))*cos(radians(lat))*
cos(radians(lng)-radians($center_lng))+sin(radians($center_lat))*sin(radians(lat))))'))
->having('distance < $radius')
->order ('distance')
->limit(0,20);
$this->view->markers = $markers->fetchAll($select);