Strange error in custom library

[eluser]Razican[/eluser]
I have changed that into this:

Code:

var $file            = 'data/online_users.tmp';
    var $data;
    var $ip;

    function Online_users()
    {
        $this->file        = APPATH.$this->file;

But it tells me this:

Code:

Use of undefined constant APPATH - assumed 'APPATH'

How can I use that constant into my library?