When using a variable variable in a class, $$this->varname does not work, ${$this->varname} must be used. 

Here is a simple script to show this in action. Put the {} in and it will work:

<?
$id = "yes";

class testClass {
	var $varname;
	
	function testClass($varname) {
		$this->varname = $varname;
	}
	
	function foo() {
		global $$this->varname;
		
		echo "$".$this->varname." is ".$$this->varname;
	}
}

$test = new testClass("id");
$test->foo();

?>