<?php
add_action( 'add_meta_boxes', 'mcdev_add_meta_to_posts' );

function mcdev_add_meta_to_posts() {
	// Add new meta box.
	add_meta_box(
		'mcdev_post_note_box',
		'Test TinyMCE Note',
		'mcev_render_meta_posts_content',
		'post',
		'normal'
	);
}

function mcev_render_meta_posts_content() {
	global $post;

	// Get saved meta data.
	$post_note_meta_content = get_post_meta( $post->ID, 'mcdev_post_note', true );

	// Hidden nonce field.
	wp_nonce_field( 'mcdev_post_note' . $post->ID, 'mcdev_post_note_nonce' );

	// Render editor metabox.
	wp_editor( $post_note_meta_content, 'mcdev_post_note', array( 'textarea_rows' => '5' ) );
}

add_action( 'save_post', 'post_note_meta_box_save' );

function post_note_meta_box_save( $post_id ) {
	// Bail if auto save.
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}

	// Bail if nonce is missing or nonce verification fails.
	if ( ! isset( $_POST['mcdev_post_note_nonce'] ) || ! wp_verify_nonce( $_POST['mcdev_post_note_nonce'], 'mcdev_post_note' . $post_id ) ) {
		return;
	}

	// Bail is current user is allowed to edit posts.
	if ( ! current_user_can( 'edit_post' ) ) {
		return;
	}

	// Make sure our data is set before trying to save it.
	if ( isset( $_POST['mcdev_post_note'] ) ) {
		update_post_meta( $post_id, 'mcdev_post_note', $_POST['mcdev_post_note'] );
	}
}