what id differences between $this->{$spec} and $this->$spec?
<p>also these two line:</p>

<pre><code>$subForm = $this-&gt;{$spec}
$subForm = $spec;

public function prepareSubForm($spec)

{
    if (is_string($spec)) {
        $subForm = $this-&gt;{$spec};
    } elseif ($spec instanceof Zend_Form_SubForm) {
        $subForm = $spec;
    } else {
        throw new Exception('Invalid argument passed to ' .
                __FUNCTION__ . '()');
    }
    $this-&gt;setSubFormDecorators($subForm)
    -&gt;addSubmitButton($subForm)
    -&gt;addSubFormActions($subForm);
    return $subForm;
}
</code></pre>
