Thanks John. It was working in very old versions of Omeka (1.5.x) so I guessed it should work on newer versions. I guess it was disabled at some point.
Yes, much much older versions of Simple Pages did let you directly write PHP in the box. For a whole variety of reasons that’s not been allowed since 2.0.
For something like a link you can probably just write it out directly. For other things, there are Shortcodes to allow a more limited set of dynamic things in the pages.