Custom tinyMCE Style

tinymce_custom_styles.php

<?php
add_filter( 'tiny_mce_before_init', 'add_frame_style' );
/*
 * Add the new tinyMCE style
*/
public static function add_frame_style( $params ) {
	$style_formats = json_decode( $params['style_formats'] );
	$style_formats[] = array(
		'title' => 'Notes',
		'block' => 'div',
		'classes' => 'post-notes',
		'wrapper' => true
	);
	$params['style_formats'] = json_encode( $style_formats );
	return $params;
}