A WordPress meta box plugin that add a custom message or notification before the actual post / page content http://w3guy.com/wordpress-meta-box-oop/

Creating WordPress Meta Box the OOP Way

<?php

/*
 Plugin Name: Custom Message Meta Box
 Plugin URI: http://sitepoint.com
 Description: Add custom message to each WordPress post
 Author: Agbonghama Collins
 Version: 1.0
 Author URI: http://tech4sky.com/
 */

/**
 * The Class.
 */
class MetaBoxClass {

	/**
	 * Hook into the appropriate actions when the class is constructed.
	 */
	public function __construct() {

		add_action('add_meta_boxes', array($this, 'cs_add_meta_box'));
		add_action('save_post', array($this, 'save'));
		add_action('the_content', array($this, 'custom_message'));
	}

	/**
	 * Adds the meta box container.
	 */
	public function cs_add_meta_box($post_type) {
		$post_types = array('post', 'page');

		//limit meta box to certain post types
		if (in_array($post_type, $post_types)) {
			add_meta_box('cs-meta',
			'Add Custom Message',
			array($this, 'cs_meta_box_function'),
			$post_type,
			'normal',
			'high');
		}
	}

	/**
	 * Render Meta Box content.
	 *
	 * @param WP_Post $post The post object.
	 */

	public function cs_meta_box_function($post) {

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

		// Use get_post_meta to retrieve an existing value from the database.
		$custom_message = get_post_meta($post -> ID, '_cs_custom_message', true);

		// Display the form, using the current value.
		echo '<div style="margin: 10px 100px; text-align: center">';
		echo '<label for="custom_message">';
		echo '<strong><p>Add custom message to post</p></strong>';
		echo '</label>';
		echo '<textarea rows="3" cols="50" name="cs_custom_message">';
		echo esc_attr($custom_message);
		echo '</textarea>';
		echo '</div>';
	}

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

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

		// Check if our nonce is set.
		if (!isset($_POST['cs_nonce_check_value']))
			return $post_id;

		$nonce = $_POST['cs_nonce_check_value'];

		// Verify that the nonce is valid.
		if (!wp_verify_nonce($nonce, 'cs_nonce_check'))
			return $post_id;

		// 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 $post_id;

		// Check the user's permissions.
		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, its safe for us to save the data now. */

		// Sanitize the user input.
		$data = sanitize_text_field($_POST['cs_custom_message']);

		// Update the meta field.
		update_post_meta($post_id, '_cs_custom_message', $data);
	}

	public function custom_message($content) {
		global $post;
		//retrieve the metadata values if they exist
		$data = get_post_meta($post -> ID, '_cs_custom_message', true);
		if (!empty($data)) {
			$custom_message = "<div style='background-color: #FFEBE8;border-color: #C00;padding: 2px;margin:2px;font-weight:bold;text-align:center'>";
			$custom_message .= $data;
			$custom_message .= "</div>";
			$content = $custom_message . $content;
		}

		return $content;
	}

}

/**
 * Finally, instantiate the class
 */

new MetaBoxClass;