WordPress/wp-content/themes/twentynineteen/inc/icon-functions.php

<?php
/**
 * SVG icons related functions
 *
 * @package WordPress
 * @subpackage Twenty_Nineteen
 * @since Twenty Nineteen 1.0
 */

/**
 * Gets the SVG code for a given icon.
 */
function twentynineteen_get_icon_svg( $icon, $size = 24 ) {
	return TwentyNineteen_SVG_Icons::get_svg( 'ui', $icon, $size );
}

/**
 * Gets the SVG code for a given social icon.
 */
function twentynineteen_get_social_icon_svg( $icon, $size = 24 ) {
	return TwentyNineteen_SVG_Icons::get_svg( 'social', $icon, $size );
}

/**
 * Detects the social network from a URL and returns the SVG code for its icon.
 */
function twentynineteen_get_social_link_svg( $uri, $size = 24 ) {
	return TwentyNineteen_SVG_Icons::get_social_link_svg( $uri, $size );
}

/**
 * Display SVG icons in social links menu.
 *
 * @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  $item_output The menu item output with social icon.
 */
function twentynineteen_nav_menu_social_icons( $item_output, $item, $depth, $args ) {
	// Change SVG icon inside social links menu if there is supported URL.
	if ( 'social' === $args->theme_location ) {
		$svg = twentynineteen_get_social_link_svg( $item->url, 26 );
		if ( empty( $svg ) ) {
			$svg = twentynineteen_get_icon_svg( 'link' );
		}
		$item_output = str_replace( $args->link_after, '</span>' . $svg, $item_output );
	}

	return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'twentynineteen_nav_menu_social_icons', 10, 4 );

/**
 * Add a dropdown icon to top-level menu items.
 *
 * @param string $output Nav menu item start element.
 * @param object $item   Nav menu item.
 * @param int    $depth  Depth.
 * @param object $args   Nav menu args.
 * @return string Nav menu item start element.
 * Add a dropdown icon to top-level menu items
 */
function twentynineteen_add_dropdown_icons( $output, $item, $depth, $args ) {

	// Only add class to 'top level' items on the 'primary' menu.
	if ( ! isset( $args->theme_location ) || 'menu-1' !== $args->theme_location ) {
		return $output;
	}

	if ( in_array( 'mobile-parent-nav-menu-item', $item->classes, true ) && isset( $item->original_id ) ) {
		// Inject the keyboard_arrow_left SVG inside the parent nav menu item, and let the item link to the parent item.
		// @todo Only do this for nested submenus? If on a first-level submenu, then really the link could be "#" since the desire is to remove the target entirely.
		$link = sprintf(
			'<button class="menu-item-link-return" tabindex="-1">%s',
			twentynineteen_get_icon_svg( 'chevron_left', 24 )
		);

		// Replace opening <a> with <button>.
		$output = preg_replace(
			'/<a\s.*?>/',
			$link,
			$output,
			1 // Limit.
		);

		// Replace closing </a> with </button>.
		$output = preg_replace(
			'#</a>#i',
			'</button>',
			$output,
			1 // Limit.
		);

	} elseif ( in_array( 'menu-item-has-children', $item->classes, true ) ) {

		// Add SVG icon to parent items.
		$icon = twentynineteen_get_icon_svg( 'keyboard_arrow_down', 24 );

		$output .= sprintf(
			'<button class="submenu-expand" tabindex="-1">%s</button>',
			$icon
		);
	}

	return $output;
}
add_filter( 'walker_nav_menu_start_el', 'twentynineteen_add_dropdown_icons', 10, 4 );