The PHP example code calls setAssertionCredentials() on an apiClient object. Only problem is, there is no such method (as far as grep & I can determine). Searching the group for that function name only turns up one message (Python-related); surely I can't be the only person using PHP?
(I'd appreciate being warned off of this project if it won't work [yet]: JS on browser contacts server, gets customized OAuth2 URL, JS accesses URL to retrieve non-public document from Cloud Storage using CORS. Any problems?)