Adding ul class with wp_list_categories Custom Walker

I am trying to achieve something that seems like it should be simple but is causing me headaches. I am listing my categories out using wp_list_categories which gives me this…

<ul>
<li>Fruit</li>
<li>Vegetables</li>
<li>Tinned Goods</li>
<li>Dairy Products</li>
</ul>

All I want to do is add a class to the ul and li using a walker function, my function looks like this….

class Walker_Simple_Example extends Walker {  
    var $db_fields = array( 'parent' => 'parent_id', 'id' => 'object_id' );  

    function start_lvl(&$output, $depth=1, $args=array()) {  
        $output .= "\n<ul class=\"product_cats\">\n";  
    }  

    function end_lvl(&$output, $depth=0, $args=array()) {  
        $output .= "</ul>\n";  
    }  

    function start_el(&$output, $item, $depth=0, $args=array()) {  
        $output .= "<li class=\"item\">".esc_attr( $item->name );
    }  

    function end_el(&$output, $item, $depth=0, $args=array()) {  
        $output .= "</li>\n";  
    }  
}  

This works fine for the li items and gives them all a class of ‘item’ but the ul class just doesn’t appear. Can anyone see where I am going wrong?

You should be extending Walker_Category not the main Walker class.

class Walker_Simple_Example extends Walker_Category {  

    function start_lvl(&$output, $depth=1, $args=array()) {  
        $output .= "\n<ul class=\"product_cats\">\n";  
    }  

    function end_lvl(&$output, $depth=0, $args=array()) {  
        $output .= "</ul>\n";  
    }  

    function start_el(&$output, $item, $depth=0, $args=array()) {  
        $output .= "<li class=\"item\">".esc_attr( $item->name );
    }  

    function end_el(&$output, $item, $depth=0, $args=array()) {  
        $output .= "</li>\n";  
    }  
}  

wp_list_categories(array('walker'=> new Walker_Simple_Example));

That does work now. product_class is applied to the child uls but your walker does not preserve much of the default functionality.

If you want the class assigned to the parent <ul>, this is a bit over complicated. That <ul> comes from the wp_list_categories function itself, not from the walker. Disable the “title” which you don’t seem to be using, and write in the wrapper <ul>.

echo '<ul class="product_cats">';
  wp_list_categories(array('title_li'=> false));
echo '</ul>';