You can only retrieve public posts for any account. The broad answer is that you can perform API calls to the activities.list API method as an unauthenticated app and can retrieve public posts for users with specific user IDs (1xxxxxxxxxxx rather than "me").
You can use the Google Client library for PHP to make the requests easier. Assuming you have a fully connected Google_client
object and an instantiated Google_PlusService
, you would be able to call:
$client = new Google_Client();
$client->setApplicationName('My Google+ PHP App');
// Visit https://code.google.com/apis/console?api=plus to generate your
// client id, client secret, and to register your redirect uri.
$client->setClientId('insert_your_oauth2_client_id');
$client->setClientSecret('insert_your_oauth2_client_secret');
$client->setRedirectUri('insert_your_oauth2_redirect_uri');
$client->setDeveloperKey('insert_your_simple_api_key');
$plus = new Google_PlusService($client);
$posts = $plus->activities->listActivities('106189723444098348646', 'public', array());
The example above would retrieve the first page of Larry Page's public posts.