selenium驱动程序，optgroup内的选项无法执行

I am using the PHP webdriver bindings here: http://code.google.com/p/php-webdriver-bindings/

I can select standard option elements in a <select> no problem, but when they are a child of an <optgroup> the click() method doesn't change the selected item.

The element is found (i can var_dump the details of the element). The code I am using is:

$xpath = "//select[@name='{$element}']/optgroup[@label='{$optgrp}']/option[@value='{$value}']";
$element = $this->_webdriver->findElementBy(LocatorStrategy::xpath, $xpath);
$element->click();

If I then do the below, it will fail.

$this->assertTrue($element->isSelected());