Calling functions of other classes

[eluser]Joozt[/eluser]
I am fairly new to the CondeIgniter framework.

If i read things right then it isn't possible to call a function of an other controllers in a controller. So if i have a class with functions that I want to use in Controllers where do I place it? Should I use it as model? helper? or what


Example:
Say I have a class called Lock witch checks if content is locked or not (in use by an other user.

Code:

class Lock extends ?????
{    
    /**
     * Class initialization
     */
    public function Lock()
    {
        parent::__construct();
        
        # Load path settings file
        $this->load->config('application', true);

        $this->unvalidated_dir = $this->config->item('unvalidated_dir','application');
        $this->archived_dir = $this->config->item('archived_dir','application');
        $this->converted_dir = $this->config->item('converted_dir','application');
        $this->archived_dir = $this->config->item('archived_dir','application');
        $this->thumb_dir = $this->config->item('thumb_dir','application');
        $this->usable_dir = $this->config->item('usable_dir','application');    
        $this->unvalidated_dir = $this->config->item('unvalidated_dir','application');
    }
    function index()
    {
        Lock::setLock("get_new.xml");
    }
    
    function checkLock($filename){
            $currenttime=time();
            $xmlData = file_get_contents($this->archived_dir.$filename);

                    $domDoc = new DOMDocument ();
                    $domDoc->preserveWhiteSpace = false;
                        if ($domDoc->loadXML ($xmlData))
                    {
                        //Set lock value of XML to 1
                        $nodeItems = $domDoc->getElementsByTagName("locked");    
                                                    
                        foreach ($nodeItems as $nodeItem)
                        {
                            $lock=$nodeItem->nodeValue;
                            
                        }
                        //END
                        
                        //Set lock_timestamp value to current time
                        $nodeItems = $domDoc->getElementsByTagName("locked_timestamp");    


                        foreach ($nodeItems as $nodeItem)
                        {
                            $lockTime=$nodeItem->nodeValue;
                        }
                        //END            
                    }
                    //ADD 20 min to the locktime
                    
                    
                    //Check if a file is locked
                    if($lock!=1){
                        return false;
                    }else{
                        //ADD 20 min to the locktime
                        $lockTime=$lockTime+(20*60);
                        if($currenttime>$lockTime){
                            return false;
                        }else{
                            return true;
                        }
                        
                    }
            
                      
        
    }

Now I have an other Controller class Content

Code:

<?php
class Content extends Controller {
    
    function Content(){
        
        parent::Controller();    
        
    }
    
    function index(){
          if(Lock::checkLock){
        $this->load->view('content', $page);
          }
    }
}
?>

So I want to use my Lock class in other controllers, but what is the best place to place this Class.

I am programming object oriented and want to keep things clean.

Thanks in advance