I have found website editor that suits all my needs (their URL is
http://digi.ws/digi-lite.php). It permits to edit both PHP and HTML and view it all in WYSIWYG mode. Very simple but professional. Besides it is integrated with Plesk and Ensim. Very cheap also.
If I were you I would think instead of integrating it with your future control panel. I guess it will be cheaper and possibly more effective.