Что делает выражение $this->$x?

class TestThis
{
    public $a;
    public $b;
    public function  f1($a=1, $b=2)
    {
        $this->a = $a;
        $this->$b = $b;
    }
    public function  f2($a=5, $b=6)
    {
        echo '$a = ', $a, "\n";
        echo '$b = ', $b, "\n";
        echo '$this->a = ', $this->a, "\n";
        echo '$this->b = ', $this->b, "\n";
        echo '$this->$a = ', $this->$a, "\n";
        echo '$this->$b = ', $this->$b, "\n";
    }
}
$x = new TestThis;
$x->f1();
$x->f2();

$a = 5
$b = 6
$this->a = 1
$this->b =
$this->$a =
$this->$b =