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<?php
/**
 * Display descriptions in main navigation.
 *
 * @since Twenty Fifteen 1.0
 *
 * @param string  $item_output The menu item output.
 * @param WP_Post $item        Menu item object.
 * @param int     $depth       Depth of the menu.
 * @param array   $args        wp_nav_menu() arguments.
 * @return string Menu item with possible description.
 */
function guiom_nav_description( $item_output, $item, $depth, $args ) {
	if ( 'primary' == $args->theme_location && $item->description ) {
		$item_output = str_replace( $args->link_after . '</a>', '<div class="menu-item-description">' . $item->description . '</div>' . $args->link_after . '</a>', $item_output );
	}
	
	return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'guiom_nav_description', 10, 4 );