class Base {}
class Ext extends Base {}

$o = new Base();

$eo = new Ext($o); # build the new object based on $o, /somehow/

* * * * * * * * * * * * * * * * * * * *

class Base {
 public $x;
 public function __construct($x) {
  $this->x = $x;
 }
}

class Ext extends Base {
 public function __construct(Base $b) {
  parent::__construct($b->x);
 }
}