Call a class inside another class in PHP

Hey there I'm wondering how this is done as when I try the following code inside a function of a class it produces some php error which I can't catch

public $tasks;
$this->tasks = new tasks($this);
$this->tasks->test();

I don't know why the initiation of the class requires $this as a parameter either :S

thanks

class admin
{
    function validate()
    {
        if(!$_SESSION['level']==7){
            barMsg('YOU\'RE NOT ADMIN', 0);
            return FALSE;
        }else{
            **public $tasks;** // The line causing the problem
            $this->tasks = new tasks(); // Get rid of $this->
            $this->tasks->test(); // Get rid of $this->
            $this->showPanel();
        }
    }
}
class tasks
{
    function test()
    {
        echo 'test';
    }
}
$admin = new admin();
$admin->validate();