I am authoring an LTI connector in PHP and I am running into this message when launching the external tool from a module. I'm just at the "proof of concept" stage - trying to replicate in PHP an activity from Kevin Hansen's InstructureCon LTI workshop, where we used Python. I made it through the workshop material with no trouble, but now I'm a little stuck.
I did the following:
Wrote "index.php" which prints outputs a simple HTML page w/ the POST variables passed to it.
Added App Config w/ url pointing to index.php, w/ secrets, set to Public, no domain or options.
Added the LTI to a Module as an External Tool.
I can navigate to the page outside Canvas, and I can post to the page. However, when I click the LTI link in the module, I get the message above. There is no auth at the moment. As far as I know, Canvas should just be posting data to the page and displaying resulting HTML in an iframe.
Any ideas/help are welcome.