Ok, bit of a weird one... I want to include php code on one of my pages within the content by adding something like:
Welcome to the payment page
Blash blah vlha...
And then [paymentform] resolves to a bit of code either somewhere on the filesystem or within the database and handled by a settings page in the backend.
I didn't want to use the exec-php plugin for several reasons - mainly I don't want our client messing with the php code, secondly there are limitations using that with the visual editor etc..etc..