How to redirect to index.php after checking the email if already exist in database?

Im trying to use Google OAuth 2.0 using the GSI library. Im using localhost to try to log in (authenticate) using google account. So I need to get the email address to check if already exist in the database. If yes, proceed to assigning the email address to session and redirect to index.php. If not, user will redirect to a form to complete the sign-up process.

The problem is, after I select Google account, it redirects to login.php. I tried to surf for answers but it's no use as the tutorials are using (gapi) oauth 2.0

The files are: login.php google_signin.php (where the checking of email takes place)

Here is the code for reference. Suggestions are highly appreciated

<script>
        function onSignIn(response) {
            const { id_token, access_token } = response.credential;

            
            fetch('google_signin.php', {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json',
                },
                body: JSON.stringify({ id_token, access_token }),
            })
            .then(response => response.json())
            .then(data => {
                if (data.success) {
                    
                    console.log('User is authenticated and can be logged in.');
                } else {
                    
                    console.error('User email address not found in the database:', data.email);
                }
            })
            .catch(error => {
                console.error('Server verification failed:', error);
            });
        }
        </script>

<script src="https://accounts.google.com/gsi/client" async defer></script>
<div id="g_id_onload"
     data-client_id="34178471422-5chptlf4ujd05p5a6suhvglmcc2f9jgk.apps.googleusercontent.com"
     data-callback="onSignIn"
     data-auto_prompt="false"
     data-ux_mode="redirect"
     data-login_uri="http://localhost/public_html/index.php">
</div>
<div class="g_id_signin" data-type="standard"></div

Here is the PHP

<?php
require('connect.php');

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $postdata = file_get_contents("php://input");
    $request = json_decode($postdata);

    if (isset($request->email) && isset($request->accessToken)) {
        $email = $request->email;

        // Check if email exists in database
        $query = "SELECT * FROM tblAccount WHERE email = ?";
        $stmt = mysqli_prepare($con, $query);
        mysqli_stmt_bind_param($stmt, "s", $email);
        mysqli_stmt_execute($stmt);
        mysqli_stmt_store_result($stmt);
        $num_rows = mysqli_stmt_num_rows($stmt);

        if ($num_rows > 0) {
            // The email exists in the database
            $response = ['success' => true, 'message' => 'Email is registered.'];
        } else {
            // The email is not found in the database
            $response = ['success' => false, 'message' => 'Email is not registered.'];
        }

        mysqli_stmt_close($stmt);
    } else {
        $response = ['success' => false, 'message' => 'Invalid request.'];
    }

    // Send the JSON response
    header('Content-Type: application/json');
    echo json_encode($response);
} else {
    echo 'Invalid request method.';
}
?>

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