Auth0 PHP - manually authenticating JWT idTokens

I find it baffling just how poorly documented most big projects are. Auth0 by Okta has a fair bit of cash, lots of customers, and almost completely absent documentation. Here's how to successfully authenticate a JWT supplied by Auth0. Once your user has authenticated with Auth0, they will be given an accessToken and an idToken. Only the idToken is needed for our purposes. It will look something like this: eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6ImFiYzEyMyJ9.eyJnaXZlbl9uYW1lIjoiSm8iLCJmYW1pbHlfbmFtZSI6IlRlc3QiLCJuaWNrbmFtZSI6IkpvVGVzdCIsIm5hbWUiOiJKbyBMZSBUZXN0IiwicGljdHVyZSI6Imh0dHBzOi8vZXhhbXBsZS5jb20vam8ucG5nIiwidXBkYXRlZF9hdCI6IjIwMjYtMDQtMjhUMTM6NTk6NTUuNjcxWiIsImVtYWlsX3ZlcmlmaWVkIjp0cnVlLCJpc3MiOiJodHRwczovL2V4YW1wbGUuZXUuYXV0aDAuY29tLyIsImF1ZCI6ImFiYzEyMyIsInN1YiI6ImZhY2Vib29rfDEyMzQ1NiIsImlhdCI6MTc3NzM4NDc5NiwiZXhwIjoxNzc3NDIwNzk2LCJzaWQiOiJhYmMxMjMtNDU2LWRlZmdoaWprIiwibm9uY2UiOiIxMjM0NTY3ODkwIn0.ZgnZxOOtfczLewlm_agK6mJMYetVTZrHlBlu5qzXbADlhvZB8RraVuFKmFutLZLibMQxz_RY0oh4hR…