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Cannot load a custom Virtual Table using PHP
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> Any pointers MUCH appreciated!!!

You've already answered your own question:

> The custom VT loads perfectly fine using the latest windows CLI downloaded from the SQLite website and using and an old sqlite version 3-8-1 20131017 CLI.

Ergo, the problem is related to the PHP binding and you'll need to ask the PHP people for a solution. (The "not authorized" warning suggests that your PHP build (or configuration) does not permit loading modules, and the PHP folks will be able to say how to remedy that.) This particular project is only responsible for the sqlite C library, the CLI app, and a small handful of closely-related code, not the hundred+ third-party bindings such as those produced by the PHP team.