Setting up gmail class

Looks a little odd. The ctor is used to pass information into the class when it is instantiated. Looking at your code, you seem to be implying that $service is needed to set up the class, but you don't ever use this $service. What you are doing is creating a new $service using the $gmToken and $gmReToken. That info looks like it is being retrieved from the DB. You probably need to pass some info into the class that will allow you to query the DB.

A few other things of note.

In your ctor, you seem to be looping over something (assuming a DB result). Each time over the loop and you will overwrite the values of $gmToken and $gmReToken, so by the time you get to creating the new service, you will only be using the last values. If your DB only ever returns 1 record (which I'm guessing it should), then you don't need to loop over the results - just grab the results. It would also make sense here to grab the result as an Object rather than an Array (you are working with Objects after all).

Secondly, inside a class, whenever you're setting or getting an internal class field ($service, $client, $gmToken, $gmReToken etc.), you must refer to them with $this->.

Have a look at this:

class GmailService {

    public $service;

    private $client;
    private $gmToken;
    private $refreshToken;
    private $db;

    public function __construct(int $id)
    {
        // set up your PDO connection here and store it in $this->db
        $this->db = new PDO(...);

        $this->db->prepare("SELECT token, token_refresh FROM yourTable WHERE id = ?");

        $stmt = $this->db->execute($id);
        $user = $stmt->fetch();

        $this->gmToken = $user->token;
        $this->gmReToken = $user->token_refresh;
        
        $this->client = $this->getGmClient($this->gmToken, $this->gmReToken);
        $this->service = new Google_Service_Gmail($this->client);
    }

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It'll need some work doing on it (definitely error/exception handling), but it should hopefully give you a better idea of how classes work.

Totally agree with Julian on this - Dependency Injection is the way to go regarding the DB. The idea behind dependency injection is to de-couple your class from the dependencies it needs. As I coded, your GMail class would be tightly coupled to a specific connection to the DB. By injecting the connection, you remove this coupling, allowing you better re-use of your class (you can pass whatever PDO connection in). It's a fundamental principle of OOP.

Regarding the $stmt->fetch(PDO::FETCH_OBJ), that makes perfect sense. I tend to always set this up as the default in the PDO ctor.

$options  = [
    PDO::ATTR_EMULATE_PREPARES   => false,
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_OBJ,
];

$db = new PDO($dsn, $username, $password, $options);

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This way, I'm always fetching objects by default, and I can then override if I ever need an array (yuck!). In this case it's probably right to put it in the class in case you forget to set the default in the injected PDO :)

I tend to always set this up as the default in the PDO ctor.

+1
I suspected you might be but as you did not include your PDO connection code - most people leave that out - and if they do fetch() defaults to both indexed and Assoc array - which would then be confusing.

For EE I prefer to be explicit - so if the constructor step is missed (Setting the mode) the rest of the code still works.

Thank you both for the tips and points, i will deff need to re-read but it has given me a far better scope on doing this.

thanks for the new learning

You are welcome.

No worries :)