The solution was pretty simple. The and was creating problems. It was supposed to be capital.

All of these worked:

is:unread in:random
is:unread AND label:random
label:unread label:random
label:unread AND label:random

You retrieve all threads with the label 'Inbox' and then you retrieve all messages for those threads.

Be aware that a thread contains both incoming and outgoing messages.

So it will contain the label 'Inbox' (for the incoming messages), but

if you list all messages of the thread, you will also list the sent messages.

Instead of querying

$this->gmailApi->get_threads($service,$userId,$param);

you need to use

$service->users_messages->listUsersMessages($userId, $param);

specifying labelIds[] as optional parameter for this method.

Why?

• If you query for threads with "is:unread" as q parameter and labelIds "INBOX", the results will contain all threads which contain at least one unread message in the inbox.
• If in the next step you retrieve all messages belonging to those threads, you will obtain all messages belonging to those threads - some of which might be read and/ or not in the inbox.
• Instead, you need to directly query only for all those messages (not threads) with q "is:unread" and labelIds "INBOX"

Sample code snippet based on your code:

//the following code works only if setUseBatch() is set to false
$client->setUseBatch(false);
$messagesResponse = $service->users_messages->listUsersMessages($userId, $param);
if ($messagesResponse) {  
  $messages = $messagesResponse->getMessages();
  foreach ($messages as $key => $val) {
   print ("\n unread message id ".$val->id."\n");
  }
 }