jsConnectPHP safeHeader not found

I've installed Vanilla and have it functioning. Am now trying to wire-up SSO using jsConnectPHP.
I've followed https://docs.vanillaforums.com/help/sso/jsconnect/#technical-overview.
I'm getting an exception due to the function 'safeHeader' not being defined. I note that it is defined in {Vanilla Forum root}/library/core/functions.compatibility.php.
If I include path to functions.compatibility.php, I end up with other functions not defined.
The documentation indicates that functions.jsconnect.php is the only file I need in my project to connect SSO, but that does not appear to be the case.
How do I reconcile these include path issues to ensure 'safeHeader' function is resolved.
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