Wordpress -致命错误:在非对象$wpdb上调用成员函数get_var()

[s_ha_dum在http://wordpress.stackexchange.com上的解决方案]

我正试图根据在帖子设置中输入的密码将用户引导到某个帖子。我几乎有工作代码:

页面上的表单:

<form method="post" action="">
  <input type="password" name="passwordfield">
  <input type="hidden" name="homepagepassword" value="1">
  <input type="submit" value="Submit">
</form>

functions.php中的代码

function doPasswordStuff(){
    if(isset($_POST['homepagepassword'])){
    $post_password = $_POST['passwordfield'];
    $post_id = $wpdb->get_var( $wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE post_password = %s", $post_password) );
    $q = new WP_Query( 'p=$post_id' );
    if($q->have_posts()){
        while($q->have_posts()){
            $q->the_post();
            wp_redirect(get_permalink());
            die();
        }
    } else {
        // oh dear, there isnt a post with this 'password', put a redirect to a fallback here
        wp_redirect('http://www.google.com');
        die();
    }
    wp_reset_query();
    }
}
add_action('init','doPasswordStuff');

然而，我得到致命错误(致命错误:调用非对象上的成员函数get_var())在这一行:

$post_id = $wpdb->get_var($wpdb->prepare("SELECT ID FROM $wpdb->posts .WHERE post_password = %d"， $post_password));