Hi Stephen,
I still haven't gotten this to work yet either. I had to stop work on attempting to do this because of compeeting priorities. I had chats to colleagues more proficient in PHP than me and they can't see anything wrong with the code (in the 30 minutes I had with them). Also, our Moodle instance is already using other external tools via LTI successfully, so it is also unlikely to be our Moodle instance.
all the investigation I have done points to the LTI Wordpress plugin or the environment it is sitting in, generating a different signature due to some unknown token or parameter. But I can't figure out what.
Since this is not an active developed plugin, I have provided my techncial evaluation and advice to our learning and teaching area to either find a different external tool that has better supported plugins, or comission work to write our own LTI plugin for Wordpress. However, they are very interested in using this one if we can get it to work.
Regards,
Petri