MySQL ~ Querying for locations by defined radius

switch ($unit)
{
  /*** miles ***/
  case 'm':
   $unit = 3963;
   break;
  /*** nautical miles ***/
  case 'n':
   $unit = 3444;
   break;
  default:
   /*** kilometers ***/
   $unit = 6371;
}

$sql = "SELECT name, club_id, latitude,longitude, ( ". $unit . " * ACOS( COS( RADIANS( " . $latitude. ") ) * COS( RADIANS( latitude ) ) * COS( RADIANS( longitude ) - RADIANS( ". $longitude. ") ) + SIN( RADIANS(".$latitude.") ) * SIN( RADIANS( latitude ) ) ) ) AS distance FROM clubs HAVING distance < ".$distance." ORDER BY distance";