I'm not sure if there's an easier way, but this should work:
$acl = new Google_Service_Storage_ObjectAccessControl();
$acl->setEntity('allUsers');
$acl->setRole('READER');
$acl->setBucket('<BUCKET-NAME>');
$acl->setObject('<OBJECT-NAME>');
// $storage being a valid Google_Service_Storage instance
$response = $storage->objectAccessControls->insert('<BUCKET-NAME>', '<OBJECT-NAME>', $acl);
You can see all the possible values here.
Also, this requires the https://www.googleapis.com/auth/devstorage.full_control
scope when authenticating.