In Zend_Config_Xml the __constructor's doc says:

   /**
     * Loads the section $section from the config file (or string $xml for
     * access facilitated by nested object properties.
     *
     * Sections are defined in the XML as children of the root element.
     *
     * In order to extend another section, a section defines the "extends"
     * attribute having a value of the section name from which the extending
     * section inherits values.
     *
     * Note that the keys in $section will override any keys of the same
     * name in the sections that have been included via "extends".
     *
     * @param  string  $filename           File to process
     * @param  mixed   $section            Section to process
     * @param  boolean $allowModifications Wether modifiacations are allowed at runtime
     * @throws Zend_Config_Exception When filename is not set
     * @throws Zend_Config_Exception When section $sectionName cannot be found in $filename
     */

But the function declaration is: public function __construct($filename, $section = null, $options = false)