Gmail API php listUserMesages() null - php

Question

I am doing a very basic test of Gmail API using PHP. I am using the code supplied by Google (after a successful authorization via OAuth followed by a successful -- code 200 -- test run on the API workbench. My error is:
PHP Fatal error: Call to a member function listUsersMessages() on null
The line that fails:
$messagesResponse = $service->users_messages->listUsersMessages($userId, $opt_param);
The full Code:
$userId = 'my gmail id';
$service = 'my api client id, for example,12356686375';
$messages = listMessages($service,$userId);
/**
* Get list of Messages in user's mailbox.
*
* #param Google_Service_Gmail $service Authorized Gmail API instance.
* #param string $userId User's email address. The special value 'me'
* can be used to indicate the authenticated user.
* #return array Array of Messages.
*/
function listMessages($service, $userId) {
$pageToken = NULL;
$messages = array();
$opt_param = array();
do {
try {
if ($pageToken) {
$opt_param['pageToken'] = $pageToken;
}
$messagesResponse = $service->users_messages->listUsersMessages($userId, $opt_param);
if ($messagesResponse->getMessages()) {
$messages = array_merge($messages, $messagesResponse->getMessages());
$pageToken = $messagesResponse->getNextPageToken();
}
} catch (Exception $e) {
print 'An error occurred: ' . $e->getMessage();
}
} while ($pageToken);
foreach ($messages as $message) {
print 'Message with ID: ' . $message->getId() . '<br/>';
}
return $messages;
}

Answer 1

As mentioned by #Morfinismo, $service variable should have the Gmail service initialized and not the client id.
It should be:
// Get the API client and construct the service object.
$client = getClient();
$service = new Google_Service_Gmail($client);
You may follow this quickstart.