More complex classes which's properties should become protected may need to modify / initializes some of their management properties in constructor or in an important member method.

Imagine a central factory manager using a list of meta objects . Or a public/private key manager class, which has to manager 3 lists, public keys, private keys and passphrases.

If the list aren't hard-coded in each class but list objects as members are used, a constructor like
class CertMgr
{
var $PublicKeys;
var $PrivateKeys;

function CertMgr()
{
  $this->PublicKeys = & new List();
  $this->PrivateKeys = & new List();
  $this->PublicKeys->SetValue( "mine", "abcdefg" );
  $this->PrivateKeys->SetValue( "mine", "secret-of-abcdefg" );
}

function __get( $pn, &$pv )
{
  if( $this->PublicKeys->KeyExists( $pn ) )
  {
    $pv = $this->PublicKeys->GetValue( $pn );
    return TRUE;
  }
  else
  {
    return FALSE;
  }
}

} // end of class CertMgr

overload( "CertMgr" );

$myCertMgr = new CertMgr(); // will fail!