Description:
------------
why section 1 is error and section 2 is correct?

Test script:
---------------
section 1:
class p {
    public function a() {
        echo '123<hr>';
        $p = 'this';
        return $$p;
    }
}
$c = new p();
$c->a();

section 2:
class p {
    public function a() {
        echo '123<hr>';
        $this;
        $p = 'this';
        return $$p;
    }
}
$c = new p();
$c->a();

Expected result:
----------------
i deem section 1 should be no error,while it give me a notice.