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 <<<html
 这一整段代码的作用是什么

php的?定界符
http://bbs.csdn.net/topics/390024612