Google Sign In for Websites causing "Signature verification failed" with JWT PHP library

Google web sign in has driven me positively crazy...

I'm building a simple web application, and I'm trying to integrate Google's sign in feature into the website (https://developers.google.com/identity/sign-in/web/).

The JavaScript seemed to go fairly well, and the next step was to verify the id_token I was receiving with my backend server (again, against Google's recommendation: https://developers.google.com/identity/sign-in/web/backend-auth).

It's a PHP-based web application, and I've successfully installed the Google Client API library using composer: composer require google/apiclient, but when posting my id_token value to my PHP backend system I'm consistently receiving the following error:

Firebase\JWT\SignatureInvalidException
File: .\vendor\firebase\php-jwt\src\JWT.php:112 
Message: Signature verification failed

Stack trace:
#0 .\vendor\google\apiclient\src\Google\AccessToken\Verify.php(103): Firebase\JWT\JWT::decode('eyJhbGciOiJSUzI...', '-----BEGIN PUBL...', Array)
#1 .\vendor\google\apiclient\src\Google\Client.php(712): Google_AccessToken_Verify->verifyIdToken('eyJhbGciOiJSUzI...', '10...')

I've also used the id_token value on Google's "tokeninfo" endpoint (https://www.googleapis.com/oauth2/v3/tokeninfo?id_token=ABC123), and the id_token validates perfectly, so I'm sure it's not the id_token value that's wrong. It's also passing it perfectly via the POST variable to the PHP script, so I'm at a bit of a loss.

Here's my code:

Javascript:

<script src="https://apis.google.com/js/platform.js?onload=googleAppStart" async defer></script>
<script>
    var googleAppStart  = function(){gapi.load('auth2', initGoogleSignIn);};
    var auth            = false;
    function initGoogleSignIn(){
        auth = gapi.auth2.init({
            client_id   : 'client-id-is-here',
            scope       : 'profile'
        });

        auth.attachClickHandler(document.getElementById('my-button'));
        auth.isSignedIn.listen(googleSignInChanged);
        auth.currentUser.listen(googleCurrentUserChanged);

        if (auth.isSignedIn.get() == true)
            auth.signIn();
    }

    function googleSignInChanged(signed_in){}
    function googleCurrentUserChanged(user){
        var auth_response   = user.getAuthResponse();
        var id_token        = auth_response.id_token;
        if (id_token != undefined){
            var url     = '/verify-google-signin';
            var params  = {id_token: id_token};
            jQuery.post(url, params, function(data){}, 'json');
        }
    }
</script>

...and my PHP catching the POST:

<?php

    require_once '/vendor/autoload.php';

    $credentials = array("client_id" => "client-id-is-here");
    $client = new \Google_Client($credentials);

    $data = $_POST;
    if (isset($data['id_token'])) {

        $id_token = trim($data['id_token']);

        // Exception generated here...
        $payload = $client->verifyIdToken($id_token);
    }

?>

Thank you so much for taking the time to read this, and for any assistance! It's greatly appreciated!