Access private props through ancestor

I have a situation where I want to write an extensible class that is
capable of saving / restoring properties of classes derived from it.
A simplified example is explained as follows;-

class A
{
private $Save ;

public function Push()
{
$this->Save = serialize( get_object_vars( $this ) ) ;
}

public function Pop()
{
foreach( unserialize( $this->Save ) as $Prop => $Value )
$this->$Prop = $Value ;
}
}

class B extends A
{
private $Priv ;

public function SetPriv( $Num )
{
$this->Priv = $Num ;
}

public function GetPriv()
{
return $this->Priv ;
}
}

$C = new B ;
$C->SetPriv( 1 ) ;
$C->Push() ;
$C->SetPriv( 2 ) ;
echo $C->GetPriv()."<br/>\n" ;
$C->Pop() ;
echo $C->GetPriv()."<br/>\n" ;

At the point of the call to $C->Pop(), I get the following error;-

Cannot access private property B::$Priv

It is only private properties which cause this problem, public and
protected properties defined in B will save and restore as intended.

Is there a way around this ?
TIA

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