Bad Request while accesing Gmail API - php

Question

I'm trying to connect to Gmail but server says:
Uncaught exception 'Google_Service_Exception' with message 'Error calling
GET https://www.googleapis.com/gmail/v1/users/{test}%40gmail.com/messages:
(400) Bad Request' in C:\...\google-api-php-client\src\Google\Http\REST.php on line 110
I can't seem to find the problem. Here's the code:
$google_accounts = $this->getGoogleAccounts();
if (count($google_accounts) > 0) {
require_once $_SERVER['DOCUMENT_ROOT'] . '/include/google-api-php-client/src/Google/autoload.php';
require_once $_SERVER['DOCUMENT_ROOT'] . '/include/google-api-php-client/src/Google/Client.php';
require_once $_SERVER['DOCUMENT_ROOT'] . '/include/google-api-php-client/src/Google/Service/Gmail.php';
$scopes = array(
'https://mail.google.com',
'https://www.googleapis.com/auth/gmail.readonly',
'https://www.googleapis.com/auth/gmail.modify',
);
$key_file_location = $_SERVER['DOCUMENT_ROOT'] . '/include/google-api-php-client/src/keys/';
foreach ($google_accounts as $one_account) {
if (!empty($one_account->client_id) && !empty($one_account->service_mail) && !empty($one_account->key_file)) {var_dump($one_account);
$key = file_get_contents($key_file_location . $one_account->key_file);
$client = new Google_Client();
$cred = new Google_Auth_AssertionCredentials($one_account->service_mail, $scopes, $key);
$client->setAssertionCredentials($cred);
$client->setClientId($one_account->client_id);
$client->setAccessType('offline_access');
$service = new Google_Service_Gmail($client);
$opt_param = array();
$messagesResponse = $service->users_messages->listUsersMessages($one_account->login, $opt_param);
var_dump($messagesResponse);
}
}
return $list;
}
else {
return false;
}

Answer 1

You need to add:
$cred->sub = $userEmail;
right after creating $cred where $userEmail is the email address of the user you are trying to impersonate. For reference, see the Google Drive Service Account documentation which uses a different scope and API call but is otherwise similar to what you'd want to do with Gmail API.