<?php $myclass->foo['bar'] = 'baz'; ?>
 
When overriding __get and __set, the above code can work (as expected) but it depends on your __get implementation rather than your __set. In fact, __set is never called with the above code. It appears that PHP (at least as of 5.1) uses a reference to whatever was returned by __get. To be more verbose, the above code is essentially identical to:
 
<?php
$tmp_array = &$myclass->foo;
$tmp_array['bar'] = 'baz';
unset($tmp_array);
?>

Therefore, the above won't do anything if your __get implementation resembles this:

<?php 
function __get($name) {
    return array_key_exists($name, $this->values)
        ? $this->values[$name] : null;
}
?>

You will actually need to set the value in __get and return that, as in the following code:

<?php
function __get($name) {
    if (!array_key_exists($name, $this->values))
        $this->values[$name] = null;
    return $this->values[$name];
}
?>