WP shortcode: Bootstrap 4 dropdown menu button

wp-bootstrap-dropdown-menu-shortcode.php

<?php
/**
 * 'button-menu' shortcode
 * - insert a simple dropdown button menu 
 * - ref: https://getbootstrap.com/docs/4.2/components/dropdowns/
 * - 'menu' parm should match a menu location
 * - e.g., register_nav_menu( 'learning_grants', 'Learning Enrichment Grants' );
 */
add_shortcode( 'button-menu', function( $atts, $content=null ) {
	extract( shortcode_atts(
		array(
			'menu' => '',
			'title' => 'Select item',
			'class' => 'btn btn-primary',
			'id' => 'dropdownMenuButton',	// only needs changing if multiple on page
		), 
		$atts 
	));

	$menu_items = wp_get_nav_menu_items( $menu );
	if ( !$menu_items ) {
//		mcw_log( "can't find $menu" );
		return;
	}
	$this_ID = get_the_ID();
    ob_start();
?>
<div class="button-menu-shortcode">
  <button class="<?= $class;?> dropdown-toggle" type="button" id="<?= $id;?>" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    <?= $title;?>
  </button>
  <div class="dropdown-menu" aria-labelledby="<?= $id;?>">
<?php
	foreach ( $menu_items as $key => $item ) {
		$active_class = ( $item->object_id == $this_ID ) ? ' active' : '';
?>
    <a class="dropdown-item<?= $active_class;?>" href="<?= $item->url;?>"><?= $item->title;?></a>
<?php
	}
?>
  </div>
</div>
<?php
    return ob_get_clean();
});

see example of use near the bottom of this page: https://cpsfoundation.org/grants-awards/learning-enrichment-grants/