class MyClass
{  var $Database = 0, $Tree = 1;

   // Returns the previous selection or zero if tree is empty
   cFunction SetTree( $newtree )
   {  $rv = $this->$Tree;
      $this->$Tree = $newtree;
      return $rv;
   }
}

Before the call:
$Database = 1
$Tree = 1

Call: $this->SetTree( $tree ) where $tree = "test"

After the line:  $this->$Tree = $newtree
$Database = test
$Tree = 1

This one's more of a stickler.
Chris