看起来您的换行符已被删除。正因为如此,从第 17 行开始的注释// don't output children opening tag实际上会继续贯穿代码的其余部分。您需要添加适当的换行符。
<?php
// Activate menu function
add_action('init', 'my_custom_menus');
function my_custom_menus()
{
    register_nav_menus(array(
        'main-menu' => 'Main Menu',
        'footer-menu' => 'Footer Menu',             
            'top_menu' => 'Top Menu',           
            'resp_menu' => 'Responsive Menu'        
    ));
}
// Main walker menu
class Walker_Responsive_Menu extends Walker_Nav_Menu {    
function start_lvl(&$output, $depth){      
$indent = str_repeat("\t", $depth); 
    // don't output children opening tag (`<ul>`)    
}    function end_lvl(&$output, $depth){      
    $indent = str_repeat("\t", $depth); 
    // don't output children closing tag    
}       
function start_el(&$output, $item, $depth, $args) {     
    global $wp_query;               
    $item_output = $attributes = $prepend ='';      
    // Create a visual indent in the list if we have a child item.      
    $visual_indent = ( $depth ) ? str_repeat('    ', $depth) : '';      
    // Load the item URL        
    $attributes .= ! empty( $item->url ) ? ' value="'   . esc_attr( $item->url ) .'"' : '';     
    // If we have hierarchy for the item, add the indent, if not, leave it out.     
    // Loop through and output each menu item as this.      
    if($depth != 0) 
    {           
        $item_output .= '<option ' . $attributes .'>'. $visual_indent . $item->title. '</option>';  
    } 
    else 
    {           
        $item_output .= '<option ' . $attributes .'>'.$prepend.$item->title.'</option>';        
    }       
    // Make the output happen.      
    $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args ); }}
    class description_walker extends Walker_Nav_Menu{      
        function start_el(&$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 = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );           $class_names = ' class="'. esc_attr( $class_names ) . '"';  
            $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        ) .'"' : '';           
            $prepend = '<strong>';           
            $append = '</strong>';           
            $description  = ! empty( $item->description ) ? '<span>'.esc_attr( $item->description ).'</span>' : '';           
            if($depth != 0)           
            {                     
                $description = $append = $prepend = "";           
            }            
                $item_output = $args->before;            
                $item_output .= '<a'. $attributes .'>';            
                $item_output .= $args->link_before .$prepend.apply_filters( 'the_title', $item->title, $item->ID ).$append;            
                $item_output .= $description.$args->link_after;            
                $item_output .= '</a>';            
                $item_output .= $args->after;            
                $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );            
        }
    }   
?>