Wordpress walker call parent walk() method - php

Question

When extending the base Walker class I need to extend the walk() method.
However, calling the parent walk() method yields no results.
These are the approaches I have tried:
public function walk($elements, $max_depth) {
parent::walk($elements, $max_depth);
}
public function walk($elements, $max_depth) {
$parent_class=get_parent_class($this);
$args = array($elements, $max_depth);
call_user_func_array(array($parent_class, 'walk'), $args);
}
It appears to me that as soon as I override the walk() things break.
Should this method return some specific value?
Should I call the parent method differently?

Answer 1

Walker::walk will return the string resulting from the walk operation.
What you will get is a text that has been created using the methods Walker::display_element, Walker::start_lvl, Walker::start_el and so on...
What you will get from the parent method is already HTML code probably hard to modify in the right way in a second time, but if you really want to do that:
public function walk($elements, $max_depth) {
$html = parent::walk($elements, $max_depth);
/* Do something with the HTML output */
return $html;
}

Answer 2

As pointed out in a comment by #TheFallen, the class Walker of Wordpress gives back an output
// Extracted from WordPress\wp-includes\class-wp-walker.php
public function walk( $elements, $max_depth ) {
$args = array_slice(func_get_args(), 2);
$output = '';
//invalid parameter or nothing to walk
if ( $max_depth < -1 || empty( $elements ) ) {
return $output;
}
...
So, if you want to extend the class and overwrite the method, you MUST keep the original behaviour, returning the output too. My suggestion:
class Extended_Walker extends Walker {
public function walk( $elements, $max_depth ) {
$output = parent::walk($elements, $max_depth);
// Your code do things with output here...
return $output;
}
}