php-return<<<html这段代码是什么意思？

public static function buildFullPage($data){    $sections = "";    $GLOBALS['tid'] = $data['tid'];    $moduleList = $data['moduleList'] ? $data['moduleList'] : $data['module_list'];    $css = base64_decode($data['css']);    usort($moduleList,'self::sortModuleList');    if(!$moduleList)return null;    foreach($moduleList as $module){        if($module['del']==1)break;        $json = json_decode(base64_decode($module['content']),true);        $sections .= self::moduleHandler( $json );    }    $config = HaloEnv::getConfig();    $resourceRoot = $config['img']['root'];    return <<

 这一整段代码的作用是什么