wordpress wpdb类确实很强大，但在实际运用中很容易出错，一定注意：

如果想查询数据库返回的行数，官方文档给的案例

$user_count = $wpdb->get_var($wpdb->prepare("SELECT COUNT(*) FROM $wpdb->users;"));
echo '<p>User count is ' . $user_count . '</p>';

运行后你会发现报错：Warning: Missing argument 2 for wpdb::prepare(), called in E:\ComsenzEXP\wwwroot\90xyy\wp-content\themes\90xyy\page-xuexiao.php on line 24 and defined inE:\ComsenzEXP\wwwroot\90xyy\wp-includes\wp-db.php on line 1246

百度找不到答案，google一下才知道原来是少了参数，改一下代码即可正常输出：

$user_count = $wpdb->get_var($wpdb->prepare("SELECT COUNT(*) FROM $wpdb->users;",''));
echo '<p>User count is ' . $user_count . '</p>';