following code runs properly in php3 but failed at line 3 in php4

<?php
class test {	
	var $test = $PHP_SELF;
	var $test1 = "ok";
	
	function show_test()
	{
		echo $this->test;
		echo $this->test1;
	}
}
$testing = new test;
$testing->show_test();
?>
the error shows:
Parse error: parse error in /home/chedong/public_html/test2.php on line 3
i wrote this for i find some classes i wrote runs properly in php3 but failed when it under php4.............