[WordPress] Demonstrates how to separate logic for cleaner code and maintainability when working with WordPress meta boxes.

acme-custom-meta-box.php

<?php

add_action( 'add_meta_boxes', 'acme_custom_meta_box' );
/**
 * Defines a custom post meta box that displays directly below the
 * post editor in the WordPress dashboard.
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 */
function acme_custom_meta_box() {

	add_meta_box(
		'acme-custom-meta-box',
		'Acme Meta Box',
		'acme_render_meta_box',
		'post',
		'normal',
		'core'
	);

}

acme-display-the-content.php

<?php

add_filter( 'the_content', 'acme_display_the_content' );
/**
 * Determines whether or not the content for the current post should be displayed.
 * 
 * If the post meta data for 'acme-hide-content' has been set, then the content
 * will be replaced by a default message; otherwise, the default content
 * will be displayed.
 * 
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 * @param   string    The content of the post to be displayed.
 * @return  string    The content, if the associated meta data is not set; a single message, otherwise.
 */
function acme_display_the_content( $content ) {

	$should_hide_content = get_post_meta( get_the_ID(), 'acme-hide-content', TRUE );

	if ( 'on' === $should_hide_content ) {
		$content = __( 'This content is not available.' );
	}

	return $content;

}

acme-meta-box-display-2.php

<?php

/**
 * Renders the post meta box checkbox option.
 *
 * The meta box gives the user the ability to toggle the visibility of the content
 * on the front-end of the blog. This file provides the markup for the meta box
 * as well as the security nonce for ensuring the data can be saved by the
 * approprite user.
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 *
 * @package WordPress
 */

?>
<div class="meta-options">

	<!-- We include the `input` and the text within the label so the user can click the label to trigger the checkbox. -->
	<label for="acme-hide-content">
		<input type="checkbox" id="acme-hide-content" name="acme-hide-content" <?php checked( get_post_meta( get_the_ID(), 'acme-hide-content', true ), 'on', true ); ?> />
		Hide the content from displaying on this post?
	</label>

	<?php
		// We must define a nonce for security purposes. This will be validated in Acme_Meta_Box_Helper.
		wp_nonce_field( 'acme-hide-content', 'acme-hide-content-nonce' );
	?>

</div><!-- .meta-options -->

acme-meta-box-display.php

TODO.

acme-meta-box-helper.php

<?php
/**
 * Defines functionality for serializing the option saved in the Acme Post Meta Box
 *
 * The Acme Post Meta Box provides the user the option to toggle the display of the
 * post content on the front-end of the blog.
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 *
 * @package WordPress
 */

/**
 * Defines functionality for serializing the option saved in the Acme Post Meta Box.
 *
 * The Acme Post Meta Box provides the user the option to toggle the display of the
 * post content on the front-end of the blog. This class helps to register the
 * necessary hooks and validation functions to make sure the data can be saved
 * (or removed) based on the user input.
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 *
 */
class Acme_Meta_Box_Helper {

	/**
	 * Initializes the class by defining the callback for the save_post hook.
	 */
	public function __construct() {
		add_action( 'save_post', array( $this, 'save_post' ) );
	}

	/**
	 * If the current user can save the meta data associated with the specified post, then
	 * updates the meta data based on the specified value.
	 *
	 * Specifically, if the user has clicked on the option to hide the post content,
	 * then the post meta data will include that information; otherwise, the meta data
	 * will be deleted.
	 *
	 * @param    integer    $post_id    The current post being saved
	 */
	public function save_post( $post_id ) {

		if( $this->user_can_save( $post_id, 'acme-hide-content-nonce', 'acme-hide-content' ) ) {

			if ( isset( $_POST['acme-hide-content'] ) ) {
				update_post_meta( $post_id, 'acme-hide-content', $_POST['acme-hide-content'] );
			} else {
				delete_post_meta( $post_id, 'acme-hide-content' );
			}

		}

	}

	/**
	 * Verifies that the user who is currently logged in has permission to save the data
	 * from the meta box to the database.
	 *
	 * @param    integer    $post_id    The current post being saved.
	 * @param    string     $nonce      The number used once to identify the serialization value
	 * @param    string     $action     The source of the action of the nonce being used
	 * @return   boolean                True if the user can save the information
	 */
	private function user_can_save( $post_id, $nonce, $action ) {

	    $is_autosave = wp_is_post_autosave( $post_id );
	    $is_revision = wp_is_post_revision( $post_id );
	    $is_valid_nonce = ( isset( $_POST[ $nonce ] ) && wp_verify_nonce( $_POST[ $nonce ], $action ) );

	    return ! ( $is_autosave || $is_revision ) && $is_valid_nonce;

	}

}

render-acme-post-meta-box-version-1.php

<?php

/**
 * Renders the content to be displayed in the "Acme Meta Box."
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 */
function acme_render_meta_box() {
	echo "TODO";
}

render-acme-post-meta-box-version-2.php

<?php

/**
 * Renders the content to be displayed in the "Acme Meta Box" by requiring
 * the file that is responsible for rendering the content.
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 */
function acme_render_meta_box() {
	require_once get_template_directory() . '/acme-meta-box-display.php';
}

setup-acme-meta-box-helper.php

<?php

add_action( 'init', 'setup_acme_meta_box_helper' );
/**
 * Imports and instantiates the Acme Meta Box Helper which is used to make sure
 * that the post meta data can be saved that's represented by the post meta box.
 *
 * @link    http://tommcfarlin.com/wordpress-meta-boxes-aiming-for-simplicity/
 */
function setup_acme_meta_box_helper() {

	/**
	 * Includes the Acme Meta Box Helper class which is used to help serialize information in the
	 * save_post action of the post serialization life cycle.
	 */
	require_once get_template_directory() . '/class-acme-meta-box-helper.php';

	// Instantiate the helper so that it performs all necessary work when the save action fires.
	$helper = new Acme_Meta_Box_Helper();

}