Setting up a PHP OAuth Client

Our firm usually recommends PHPLeague's OAuth2 client for PHP integrations. There's is sample code in the README.md, but basically

Include the relevant Client Key and Secret in the $Provider element, i.e.

  $provider = new \League\OAuth2\Client\Provider\GenericProvider([
  'clientId'                => 'fake-id-741648bdf4e3ffc8e1e3607898e16870',
  'clientSecret'            => 'fake-secret-548755e9e3717974f7a378cd25b24e85',
  'redirectUri'             => 'https://app.example.com/callback.php',
  'urlAuthorize'            => 'https://oauth.server.com/oauth2/authz/',
  'urlAccessToken'          => 'https://oauth.server.com/oauth2/access/',
  'urlResourceOwnerDetails' => 'https://oauth.server.com/userinfo/',
]);

Make the call to getAuthorizationURL() with the $Provider, which will redirect the user to the login page, and return back to your code with ?code=[stuff]. To include OIDC support, make sure you specify the OIDC scope (as well as any others that you care about):
```
$options = [
  'scope' => ['openid profile email']
];
```
and call getAuthorizationUrl with the $options array, like:
```
$authorizationUrl = $provider->getAuthorizationUrl($options);
```
You can then use the value in $_GET['code'] to get an access token from the identity provider, like:
```
$accessToken = $provider->getAccessToken('authorization_code', [
  'code' => $_GET['code']
]);
```

using that token you'll be able to use getAuthenticatedRequest's along with the $accessToken as needed if you need to pull something from a secured API on the identity server:

$request = $provider->getAuthenticatedRequest(
  'GET',
  'https://oauth.server.com/some-api-endpoint',
  $accessToken
);
$entitlementResponse = $provider->getParsedResponse($request);

you can also just user information about the $resourceOwner that is returned (because of the scopes that we chose:
```
$resourceOwner = $provider->getResourceOwner($accessToken)->toArray();
```

The access token has expiry information, etc. and there are functions for refreshing the token as needed. All in all it's really simple to do with PHP.

I avoid JS for OAuth, but I'm pretty sure that's just a personal preference. There are good javascript libraries as well.

Note: Loosely based off of this StackOverflow answer from the author.