Adds dropdown menu of navigation menus from the Wordpress menu system to page edit screens.

siderbar-metabox.php

<?php

/**
 * Adds a box to the main column on the Post and Page edit screens.
 */
function myplugin_add_meta_box() {

	$screens = array( 'page' );

	foreach ( $screens as $screen ) {

		add_meta_box(
			'myplugin_sectionid',
			__( 'Custom Sidebar Menu', 'myplugin_textdomain' ),
			'myplugin_meta_box_callback',
			$screen
		);
	}
}
add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );

/**
 * Prints the box content.
 * 
 * @param WP_Post $post The object for the current post/page.
 */
function myplugin_meta_box_callback( $post ) {

	// Add an nonce field so we can check for it later.
	wp_nonce_field( 'myplugin_meta_box', 'myplugin_meta_box_nonce' );

	/*
	 * Use get_post_meta() to retrieve an existing value
	 * from the database and use the value for the form.
	 */
	$value = get_post_meta( $post->ID, 'sidebar_menu_name', true );

	?>
		<p>Menus can be created in the WordPress menu system: Appearance -> Menus.</p>
		<p>Once you create a menu, you can select it here.</p>
		<p>This is an optional sidebar menu. For this menu to display you must set the current page to use the Sidebar Template.</p>
		<select name="gurustu-sidebar-menu-name" id="gurustu-sidebar-menu-name">
			<option></option>
			<?php  

				$menus = get_terms( 'nav_menu', array( 'hide_empty' => false ) );
				foreach ( $menus as $menu ) {

					if( $value == $menu->name ){
						echo '<option selected>'.$menu->name.'</option>';
					} else {
						echo '<option>'.$menu->name.'</option>';
					}

				}

			?>
		</select>

	<?php
	// echo '<label for="myplugin_new_field">';
	// _e( 'Description for this field', 'myplugin_textdomain' );
	// echo '</label> ';
	// echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="' . esc_attr( $value ) . '" size="25" />';
}

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function myplugin_save_meta_box_data( $post_id ) {

	/*
	 * We need to verify this came from our screen and with proper authorization,
	 * because the save_post action can be triggered at other times.
	 */

	// Check if our nonce is set.
	if ( ! isset( $_POST['myplugin_meta_box_nonce'] ) ) {
		return;
	}

	// Verify that the nonce is valid.
	if ( ! wp_verify_nonce( $_POST['myplugin_meta_box_nonce'], 'myplugin_meta_box' ) ) {
		return;
	}

	// If this is an autosave, our form has not been submitted, so we don't want to do anything.
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}

	// Check the user's permissions.
	if ( isset( $_POST['post_type'] ) && 'page' == $_POST['post_type'] ) {

		if ( ! current_user_can( 'edit_page', $post_id ) ) {
			return;
		}

	} else {

		if ( ! current_user_can( 'edit_post', $post_id ) ) {
			return;
		}
	}

	/* OK, it's safe for us to save the data now. */
	
	// Make sure that it is set.
	if ( ! isset( $_POST['gurustu-sidebar-menu-name'] ) ) {
		return;
	}

	// Sanitize user input.
	$my_data = sanitize_text_field( $_POST['gurustu-sidebar-menu-name'] );

	// Update the meta field in the database.
	update_post_meta( $post_id, 'sidebar_menu_name', $my_data );
}
add_action( 'save_post', 'myplugin_save_meta_box_data' );