Collection class of PHP

header("Content-type:text/html; charset=utf-8");

class Collection{
    private $_members=array();
    private $_onload;
    private $_isLoaded=false;

    public function startLoad(){
        $this->_checkCallback();
    }

    public function addItem($obj,$key=NULL){
        $this->_checkCallback();
        if($key){
            if(isset($this->_members[$key])){
                throw new Exception("Key \"$key\" already in use!");
            }else{
                $this->_members[$key]=$obj;
            }
        }else{
            $this->_members[]=$obj;
        }
    }

    public function removeItem($key){
        $this->_checkCallback();
        if(isset($this->_members[$key])){
            unset($this->_members[$key]);
        }else{
            throw new Exception("Invalid key \"$key\"!");
        }
    }

    public function getItem($key){
        $this->_checkCallback();
        if(isset($this->_members[$key])){
            return $this->_members[$key];
        }else{
            throw new Exception("Invalid key \"$key\"!");
        }
    }

    public function keys(){
        $this->_checkCallback();
        return array_keys($this->_members);
    }

    public function length(){
        $this->_checkCallback();
        return sizeof($this->_members);
    }

    public function exists($key){
        $this->_checkCallback();
        return (isset($this->_members[$key]));
    }

    public function setLoadCallback($functionName,$objOrClass=NULL){
        if($objOrClass){
            $callback=array($objOrClass,$functionName);
        }else{
            $callback=$functionName;
        }

        if(!is_callable($callback)){
            /*throw new Exception("$callableName is not callable"."as a parameter to oonload");*/
            echo "$callback is not callable";
            return false;
        }

        $this->_onload=$callback;
    }

    private function _checkCallback(){
        if(isset($this->_onload)&&!$this->_isLoaded){
            $this->_isLoaded=true;
            call_user_func($this->_onload,$this);
        }
    }
}

PHP, Collection