How can I verify firebase token server side?

Question

I have a firebase account where I manually create the users who will be abled to use my site (sign up is not public, but it is not relevant to this inquiry)

In the login page, the authentication is through javascript.
Once the user enters their mail and password the corresponding function is executed, I get the token and I send it to my PHP server via url redirection. Something like this:

firebase.auth().signInWithEmailAndPassword(inputemail, inputpassw)
    .then( function(user) {

        myEmail = user.email;
        myUid = user.uid;

        user.getIdToken()
            .then( function(token){
                myToken = token;

                location.href = "http://www.example.com/verify?email="+myEmail+"&token="+myToken+"&uid="+myUid;

            });

    }, function (error) {
       ...
    }); 

Then, on my server I will have the mail, the uid, and the token.
So, my question is:
How do I verify that the token is valid? It is impossible?
I know the token is encrypted, but the key is public... so anyone could make a valid token!
I mean, for instance, I have an expired token, I can decode it, change the expiration time, encode it again and gain access to my server without knowing any password

Is there something I'm missing?

Apparently I can not verify the token via REST.
What alternative do I have?

Answer 1

From the Firebase Documentation:

The Firebase Admin SDK has a built-in method for verifying and decoding ID tokens. If the provided ID token has the correct format, is not expired, and is properly signed, the method returns the decoded ID token. You can grab the uid of the user or device from the decoded token.

So, you do not need to worry about someone trying to generate fake tokens.

To verify the token in PHP, as described in the docs Firebase Admin SDK for PHP

Minimal code for verifying the token:

use Kreait\Firebase;
use Firebase\Auth\Token\Exception\InvalidToken;

//create Firebase factory object
//$firebase = (new Firebase\Factory())->create();

//get a token from client 
//$idTokenString = 'eyJhbGciOiJSUzI1...';

try {
    $verifiedIdToken = $firebase->getAuth()->verifyIdToken($idTokenString);
} catch (InvalidToken $e) {
    echo $e->getMessage();
}

$uid = $verifiedIdToken->getClaim('sub');
$user = $firebase->getAuth()->getUser($uid);
echo $user;