我試圖將ob_start添加到我的Wordpress插件中的類中，如下所示：class ObStart {  public static function callback($buffer){    return $buffer;  }  public static function add_ob_start(){    ob_start("callback");  }  public static function flush_ob_end(){    ob_end_flush();  }}add_action('init', array( 'ObStart', 'add_ob_start' ) );add_action('wp_footer', array( 'ObStart', 'flush_ob_end' ) );這似乎不起作用，而是導致此錯誤：Warning:  ob_start(): function 'callback' not found or invalid function name in <b>\wp-content\plugins\timeline-plugin\obstart.php on line 8Notice:  ob_start(): failed to create buffer in \wp-content\plugins\timeline-plugin\obstart.php on line 8Warning:  Cannot modify header information - headers already sent by (output started at \wp-includes\formatting.php:5520) in \wp-includes\pluggable.php on line 1251Warning:  Cannot modify header information - headers already sent by (output started at \wp-includes\formatting.php:5520) in \wp-includes\pluggable.php</b> on line 1254是否可以讓ob_start在課堂上工作？如果是，我哪里出問題了？