销毁php对象引用

I will try to be succinct on this:

I have this:

Example1:

$obj1 = new SimpleObj();
$obj2 = $obj1;
$obj2 = null;

Example2:

$obj1 = new SimpleObj();
$obj2 = clone $obj1;
$obj2 = null;

Example3:

$obj1 = new SimpleObj();
$obj2 =& $obj1;
$obj2 = null;

I did a lot of research on this, but i'm not assured yet. Which one will actually destroy the object and free memory?

Thank in advance!