add_meta_box() │ WP 2.5.0

An example of adding a custom block to the edit/create pages of posts and static pages:

// Add blocks to the main column on the post/page pages
add_action( 'add_meta_boxes', 'myplugin_add_custom_box' );

// Saving data when the post is saved
add_action( 'save_post', 'myplugin_save_postdata' );

function myplugin_add_custom_box(){
	$screens = array( 'post', 'page' );
	add_meta_box( 'myplugin_sectionid', 'Meta block name', 'myplugin_meta_box_callback', $screens );
}

// HTML code of the block
function myplugin_meta_box_callback( $post, $meta ){
	$screens = $meta['args'];

	// Use nonce for verification
	wp_nonce_field( plugin_basename(__FILE__), 'myplugin_noncename' );

	// field value
	$value = get_post_meta( $post->ID, 'my_meta_key', 1 );

	// Form fields for entering data
	echo '<label for="myplugin_new_field">' . __("Description for this field", 'myplugin_textdomain' ) . '</label> ';
	echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="'. $value .'" size="25" />';
}

function myplugin_save_postdata( $post_id ) {

	// make sure the field is set.
	if ( ! isset( $_POST['myplugin_new_field'] ) )
		return;

	// check the nonce of our page, because save_post can be called from another location.
	if ( ! wp_verify_nonce( $_POST['myplugin_noncename'], plugin_basename(__FILE__) ) )
		return;

	// if this is autosave do nothing
	if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) 
		return;

	// check user permission
	if( ! current_user_can( 'edit_post', $post_id ) )
		return;

	// Everything is OK. Now, we need to find and save the data
	// clear the value of the input field.
	$my_data = sanitize_text_field( $_POST['myplugin_new_field'] );

	// Update data in the database.
	update_post_meta( $post_id, 'my_meta_key', $my_data );
}

This example shows how to add an additional accordion element to the WordPress menu settings page.

<?php
add_action( 'admin_head-nav-menus.php', 'register_my_meta_box_accordion_nav_menus' );

function register_my_meta_box_accordion_nav_menus() {
	add_meta_box( 'my-custom-meta-box', 'My metabox', 'render_my_meta_box_accordion_nav_menus', 'nav-menus', 'side' );
}

function render_my_meta_box_accordion_nav_menus() {
	?>
	<div class="my-custom-meta-box" id="my-custom-meta-box">
		Metabox content
	</div>
	<?php
}

We get it:

Finished code that adds a new item to the navigation menu (in the accordion) and fills it, allowing you to select a media library item (files) and add it into the menu:

<?php

/**
 * Registering and adding a tab in the accordion to create a menu
 *
 * @link https://gist.github.com/nikolov-tmw/8698598
 */
add_action( 'admin_head-nav-menus.php', 'register_menu_files_metabox' );

/**
 * Replaces the link to the media page with a link to the media file itself
 *
 * @param string $link link to the media file page
 * @param string $post_id Media file ID
 *
 * @return string
 */
add_filter( 'attachment_link', 'change_attachment_link', 1000, 2 );

function register_menu_files_metabox() {
	add_meta_box('menu-media-files-metabox', 'Files', 'render_menu_files_metabox', 'nav-menus', 'side', 'default' );
}

/**
 * Displaying the "Files" metabox
 *
 * @param string $object Not used.
 * @param array $args Parameters and arguments.
 */
function render_menu_files_metabox( $object, $args ) {
	global $nav_menu_selected_id;
	$files_ext = [ 'xlsx', 'pdf', 'zip' ];

	$my_items = get_posts( [
		'numberposts'    => - 1,
		'post_type'      => 'attachment',
		'post_mime_type' => get_mime_files_extension( $files_ext ),
	] );

	$walker       = new Walker_Nav_Menu_Checklist();
	$removed_args = [
		'action',
		'customlink-tab',
		'edit-menu-item',
		'menu-item',
		'page-tab',
		'_wpnonce',
	]; ?>
	<div id="tab-files-div">
	<div id="tabs-panel-list-files" class="tabs-panel tabs-panel-active">
		<p>Only files with the extension <b><?php echo implode( ', ', $files_ext ); ?></b>.</p>

		<ul id="tab-files-checklist-pop" class="categorychecklist form-no-clear">
			<?php echo walk_nav_menu_tree( array_map( 'wp_setup_nav_menu_item', $my_items ), 0, (object) [ 'walker' => $walker ] ); ?>
		</ul>

		<p class="button-controls">
			<span class="list-controls">
				<a href="<?php
				echo esc_url( add_query_arg(
					[
						'list-files' => 'all',
						'selectall'  => 1,
					],
					remove_query_arg( $removed_args )
				) );
				?>#menu-media-files-metabox" class="select-all"><?php _e( 'Select All' ); ?></a>
			</span>

			<span class="add-to-menu">
				<input type="submit"<?php wp_nav_menu_disabled_check( $nav_menu_selected_id ); ?>
					   class="button-secondary submit-add-to-menu right" value="<?php esc_attr_e( 'Add to Menu' ); ?>"
					   name="add-tab-files-menu-item" id="submit-tab-files-div"/>
				<span class="spinner"></span>
			</span>
		</p>
	</div>
	<?php
}

/**
 * Returns mime types of file extensions
 *
 * @param array $files_ext file extensions
 *
 * @return array
 */
function get_mime_files_extension( $files_ext ) {

	// Gets the list of allowed mime types.
	$mimes = get_allowed_mime_types();

	// Search for mime type in the extension array.
	$need_mimes = [];
	foreach ( $files_ext as $file_ext ) {
		foreach ( $mimes as $type => $mime ) {
			if ( false !== strpos( $type, $file_ext ) ) {
				$need_mimes[] = $mime;
			}
		}
	}

	return $need_mimes;
}

function change_attachment_link( $link, $post_id ) {
	return wp_get_attachment_url( $post_id );
}

We get it:

This example shows how to add an extra block in OOP-style:

/**
 * Call the class on the post editing page.
 */
function call_someClass() {
	new someClass();
}

if ( is_admin() ) {
	add_action( 'load-post.php', 'call_someClass' );
	add_action( 'load-post-new.php', 'call_someClass' );
}

class someClass {

	/**
	 * Set hooks at the moment of class initialization.
	 */
	public function __construct() {
		add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
		add_action( 'save_post', array( $this, 'save' ) );
	}

	/**
	 * Adding an extra block.
	 */
	public function add_meta_box( $post_type ){

			// Set the types of posts to which the block will be added
			$post_types = array('post', 'page');

			if ( in_array( $post_type, $post_types )) {
				add_meta_box(
					'some_meta_box_name',
					__( 'Some Meta Box Headline', 'myplugin_textdomain' ),
					array( $this, 'render_meta_box_content' ),
					$post_type,
					'advanced',
					'high',
				);
			}
	}

	/**
	 * Save the data when you save the post.
	 *
	 * @param int $post_id The ID of the post that is saved.
	 */
	public function save( $post_id ) {

		/*
		 * We need to do a check to make sure that the request came from our page,
		 * because save_post can be called anywhere else.
		 */

		// check if nonce is set.
		if ( ! isset( $_POST['myplugin_inner_custom_box_nonce'] ) )
			return $post_id;

		$nonce = $_POST['myplugin_inner_custom_box_nonce'];

		// We check if nonce is correct.
		if ( ! wp_verify_nonce( $nonce, 'myplugin_inner_custom_box' ) )
			return $post_id;

		// If this is autosave do nothing.
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
			return $post_id;

		// Check user rights.
		if ( 'page' == $_POST['post_type'] ) {

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

		} else {

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

		// OK, everything is clear, you can save the data.

		// Clean up the input field.
		$mydata = sanitize_text_field( $_POST['myplugin_new_field'] );

		// Updating the data.
		update_post_meta( $post_id, '_my_meta_value_key', $mydata );
	}

	/**
	 * Auxiliary unit code.
	 *
	 * @param WP_Post $post Post object.
	 */
	public function render_meta_box_content( $post ) {

		// Add a nonce field to be checked when saving.
		wp_nonce_field( 'myplugin_inner_custom_box', 'myplugin_inner_custom_box_nonce' );

		// Retrieve existing data from the database.
		$value = get_post_meta( $post->ID, '_my_meta_value_key', true );

		// Output the form fields using the obtained data.
		echo '<label for="myplugin_new_field">';
		echo __( 'Description for this field', 'myplugin_textdomain' );
		echo '</label> ';
		echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field"';
		echo ' value="' . esc_attr( $value ) . '" size="25" />';
	}

}

The first parameter passes post data (array $_POST).

The second has an array of $callback_args arguments.

// This function adds a Block that passes additional parameters to the 
// `callback` my_metabox_callback() function.
function add_my_meta_box() {
	$var1 = "this";
	$var2 = "that";
	$callback_args = array( 'foo'=>$var1,'bar'=>$var2 );

	add_meta_box( 'metabox_id', 'Metabox Title', 'my_metabox_callback', 'page', 'normal', 'low', $callback_args );
}

// $post is an object containing data from the global array $_POST
// $metabox is an array of metabox_id, title, callback
// and the new passed arguments in the $callback_args parameter
function my_metabox_callback( $post, $metabox ){

	echo 'Last Modified: ' . $post->post_modified; // the time the post was last modified
	echo $metabox['args']['foo']; // once
	echo $metabox['args']['bar']; // two
	echo get_post_meta( $post->ID, 'my_custom_field', true ); // arbitrary field value

}

You can pass an anonymous function as a callback function to the $callback_args parameter, for example:

add_action( 'add_meta_boxes', 'add_custom_meatbox' );
function add_custom_meatbox(){
	$screens = array( 'post', 'page' );
	add_meta_box( 'wpsb', __( 'Metabox title', 'my-plugin' ), function() {
		_e( 'Metabox content', 'my-plugin' );
	}, $screens );
}

It's a little more convenient if you just need to get something out in the metabox.

This example shows how to output the metabox in an custom place: right after the header field. To do this, set your own value for the $context parameter, and then output it with the edit_form_after_title hook. You can see the whole map of hooks on the post editing page here.

add_meta_box( 'my-meta-box', 'My Meta Box', 'my_meta_box_callback', null, 'my_custom_context', 'high' );

add_action( 'edit_form_after_title', 'show_custom_meatbox' );

function show_custom_meatbox( $post ) {
	do_meta_boxes( null, 'my_custom_context', $post );
}