How to style current page menu item when using a walker

A belated answer but figured I’d contribute as this page was near the top of Google when searching for a solution to this issue. I too had overlooked the fact that WP adds the current page menu item by default (thanks patnz for pointing that out).

However I’m not happy with how bloated the markup is when including the default classes so for anyone else interested you could instead use a line of code similar to the following:

$class_names = in_array("current_page_item",$item->classes) ? ' active' : '';

For completeness – here is how it would look in your code example:

class My_Walker extends Walker_Nav_Menu
{
    function start_el(&$output, $item, $depth, $args) {
        global $wp_query;
        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value="";

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;

        $class_names = in_array("current_page_item",$item->classes) ? ' active' : '';

        $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

        $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
        $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
        $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '<span>' . $item->description . '</span>';
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }
}