PHP4ではメンバ変数の参照時にドルがついてても動く

php 
<?php

class Test{
   var $pub = "public";

   function callpro(){   
    $this->pub = "php5";
    $this->$pub = "php4";

    var_dump($this->pub);
    var_dump($this->$pub);//dollar is here
  }
}

$base = new Test();
$base->callpro();

php4 displays php4 but php5 is not it,cause runtime error is called.
php4 ignores dollar and executes.