public function sum($property) {
		$sum = 0;
		
		foreach ($this->_map as $key) {
			$item = $this->_data[$key];
			
			if (isset($item[$property])) {
				$sum += $item[$property];
			}
		}
		
		return $sum;
	}
public function sum($property) {
		$sum = 0;
		
		foreach ($this->_map as $key) {
			$item = $this->_data[$key];
			
			if (isset($item[$property])) {
				$sum += $item[$property];
			}
		}
		
		return $sum;
	}
public function sum ( $ property ) { $ sum = 0 ; foreach ( $ this -> _map as $ key ) { $ item = $ this -> _data [ $ key ] ; if ( isset ( $ item [ $ property ] ) ) { $ sum += $ item [ $ property ] ; } } return $ sum ; }
Get amount of the properties

@param  string $property
@return int
Get amount of the properties