auto-login-step-2.php

// Here's an example of the route we are creating
// https://www.yourwebsite.com/wp-json/sand_login/[email protected]&secret=6Uy11Uyl

// Register the rest route
add_action( 'rest_api_init', function () {
    register_rest_route(
        'sand_login',
        'prk_route',
        array(
            'methods' => 'GET',
            'callback' => 'pirenko_login_user',
        )
    );
});

// Function to handle the request
function pirenko_login_user() {

    // Validate input
    if (isset($_GET['secret']) && isset($_GET['username']) && $_GET['username']!="") {

        $username = $_GET['username'];
        $user = get_user_by('email', $username );

        if ($user) {

            //Check if secret key matches
            $check_user = get_user_by( 'id', $user->ID );
            if ($_GET['secret']==$check_user->secret_key) {

                //Login info is correct - let's proceed

                // Manage login cookies
                wp_clear_auth_cookie();
                wp_set_current_user ( $user->ID );
                wp_set_auth_cookie ( $user->ID );

                // Finally redirect user
                $redirect_to = get_dashboard_url($user->ID);
                wp_safe_redirect( $redirect_to );
                exit();
            }
            else {
                echo "Secret key mismatch!";
                exit();
            }

        }
        else {
            echo "Could not find user!";
            exit();
        }
    }
    else {
        echo "Parameters are missing!";
        exit();
    }
}