In PHP 4, you couldn't access a member function or properity indirectly through another object's member function or properity.. 

<?php
    $foo = new foobar();

    /* this breaks in PHP 4, even if $foo->a is an object with
       another member variable $b within it */
    echo $foo->a->b;

    /* This is how it would have to work in PHP 4 */

    $tmp = $foo->a;
    echo $tmp->b;
In PHP 5, indirect referencing is allowed 

<?php

    echo $foo->a->b;