Just what I was looking for. Does the job simply and effectively.

To clarify the usage syntax:

the php file to include must be within your joomla installation (i.e contained within your joomla root folder)

to include a php file, add the following plugin tag in your content editor:

{rdaddphp file=folder/file.php}

where 'folder' is the folder containing your php file and 'file.php' is your script to include.

Thanks for the great work!