我正在研究一个儿童主题Walker_Nav_Menu 和Walker 上课引起了所有的骚动。我的父主题覆盖Walker_Nav_Menu 具体如下：

if ( ! class_exists( \'Zen_Menu_Walker\' ) ) {

class Zen_Menu_Walker extends Walker_Nav_Menu {

    public function display_element( $element, &$children_elements, $max_depth, $depth = 0, $args, &$output ) {
        $element->has_children = ! empty( $children_elements[ $element->ID ] );
        $element->classes[]    = ( $element->current || $element->current_item_ancestor ) ? \'active\' : \'\';
        $element->classes[]    = ( $element->has_children && 1 !== $max_depth ) ? \'has-dropdown\' : \'\';

        parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }

    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        $indent = ( $depth ) ? str_repeat( "\\t", $depth ) : \'\';

        $classes   = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = \'menu-item-\' . $item->ID;

        $class_names = join( \' \', apply_filters( \'nav_menu_css_class\', array_filter( $classes ), $item, $args, $depth ) );
        $class_names = $class_names ? \' class="\' . esc_attr( $class_names ) . \'"\' : \'\';

        $id = apply_filters( \'nav_menu_item_id\', \'menu-item-\' . $item->ID, $item, $args, $depth );
        $id = $id ? \' id="\' . esc_attr( $id ) . \'"\' : \'\';

        $output .= $indent . \'<li\' . $id . $class_names . \'>\';

        $atts           = array();
        $atts[\'title\']  = ! empty( $item->attr_title ) ? $item->attr_title : \'\';
        $atts[\'target\'] = ! empty( $item->target ) ? $item->target : \'\';
        $atts[\'rel\']    = ! empty( $item->xfn ) ? $item->xfn : \'\';
        $atts[\'href\']   = ! empty( $item->url ) ? $item->url : \'\';

        $atts = apply_filters( \'nav_menu_link_attributes\', $atts, $item, $args, $depth );

        $attributes = \'\';
        foreach ( $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                //$value = ( \'href\' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= \' \' . $attr . \'="\' . $value . \'"\';
            }
        }

        $item_output = $args->before;
        $item_output .= \'<a\' . $attributes . \'>\';
        /** This filter is documented in wp-includes/post-template.php */
        $item_output .= $args->link_before . apply_filters( \'the_title\', $item->title, $item->ID ) . $args->link_after;
        $item_output .= \'</a>\';
        $item_output .= $args->after;

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

    public function start_lvl( &$output, $depth = 0, $args = array() ) {
        $output .= "\\n<ul class=\\"sub-menu dropdown\\">\\n";
    }

}
}

这在父主题中没有任何致命错误。但在子主题中，它会产生致命错误：

Notice: Indirect modification of overloaded property WP_Post::$classes has no effect in /srv/www/htdocs/wpsite/wp-content/themes/zen-theme/class.menu.php on line 45

进行了一些挖掘Stackoverflow 和Google 我找到的唯一参考文献是：

• https://stackoverflow.com/questions/13421661/getting-indirect-modification-of-overloaded-property-has-no-effect-notice
• https://stackoverflow.com/questions/10454779/php-indirect-modification-of-overloaded-property
• https://stackoverflow.com/questions/20053269/indirect-modification-of-overloaded-element-of-splfixedarray-has-no-effect
• https://gist.github.com/awshout/3943026#gistcomment-676280

  “从PHP 5.3.0开始，当您使用&；in foo（&；a）；）时，您将收到一条警告，指出“按引用传递调用时间”不受欢迎。从PHP 5.4.0开始，按引用传递调用时间已被删除，因此使用它将引发致命错误。”

  猜猜为什么会这样。。。

public function display_element( $element, &$children_elements, $max_depth, $depth = 0, $args, &$output ) {
    $element->has_children = ! empty( $children_elements[ $element->ID ] );

    if ( ! isset( $element->classes ) ) {
        $element->classes = array();
    }

    $element->classes[]    = ( $element->current || $element->current_item_ancestor ) ? \'active\' : \'\';
    $element->classes[]    = ( $element->has_children && 1 !== $max_depth ) ? \'has-dropdown\' : \'\';        

    parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
}