PHP RuntimeException：无法启动会话，因为当我尝试回显字符串时已发送标头

如何解决PHP RuntimeException：无法启动会话，因为当我尝试回显字符串时已发送标头

我在自定义 Drupal 8 模块中有两个文件：a.module 文件和 FaqSchemaService.PHP 文件。我的目的是抓取页面内容并创建 JSON-LD 模式。在 FaqSchemaService.PHP 中，我创建了 JSON-LD 模式，它可以正确验证。但是，当我尝试输出缓冲并回显结果时，我得到 RuntimeException: Failed to start the session because headers have been sent 错误。我在这里做错了什么？请帮忙！

a.module:

<?PHP

function _page_attachments_alter(&$page){
  $node = \Drupal::routematch()->getParameter('node');

  // check if node is faq page content type,excluding admin nodes from check
  if(!\Drupal::service('router.admin_context')->isAdminRoute()) {
    if($node && $node->bundle() == 'faq_page') {
      ob_start();
      echo \Drupal::service('service.name')->getFaqs();
      ob_end_flush();
    }
  }
}

FaqSchemaService.PHP:

<?PHP
use Drupal\paragraphs\Entity\Paragraph;

/**
 * Class FaqSchemaService
 */
class FaqSchemaService {
  public function getFaqs() {
    $array = [];
    $node = \Drupal::routematch()->getParameter('node');
    $target = \Drupal::entityTypeManager()->getStorage('node')->load($node->id());
    foreach($target->field_bands as $item) {
      if($item->entity && $item->entity->bundle() == 'tabs_accordion') {
        $paragraph = Paragraph::load($item->entity->id());
        foreach($paragraph->field_tab as $tab) {
          $array[] = array('@type' => 'Question','name' => json_encode($tab->entity->field_tab_title->value),'acceptedAnswer' => array('@type' => 'Answer','text' => json_encode($tab->entity->field_tab_content->value)));
        };
      }
    };

    $script = '<script type="application/ld+json">{
      "@context": "https://schema.org/","@type": "FAQPage","mainEntity": ' . json_encode($array) . '}
    </script>';

    return $script;
  }
}