403 Forbidden when creating a page with RESTful API
The tutorial example using Dev Client is probably what is throwing you off because PHP_AUTH_USER and PHP_AUTH_PW are not valid request headers for Drupal 8 REST. The tutorial does give a curl command using the --user parameter which should do the correct behavior - add an Authorization header. I demonstrated this by manually adding that request header in User login REST format.

Content-Type: application/hal+json
Accept: application/hal+json
Authorization: Basic admin_username:admin_password