How to Add Custom Buttons for WordPress TinyMCE Editor

WordPress version 3.9 uses TinyMCE version 4.0 core, an open source HTML WYSIWYG editor by Moxiecode Systems, AB. This means that all of the buttons you see on your default theme posts and page editor comes from the TinyMCE plugin. WordPress TinyMCE editor version 4 provides comes with the following changes:

How to Add Custom Buttons for WordPress TinyMCE Editor