有的扩展有依赖。

yii\web\Response 方法中

protected function sendHeaders()
{
    if (headers_sent($file, $line)) {
        return; //此处返回
        throw new HeadersAlreadySentException($file, $line);
    }
    if ($this->_headers) {
        foreach ($this->getHeaders() as $name => $values) {
            $name = str_replace(' ', '-', ucwords(str_replace('-', ' ', $name)));
            // set replace for first occurrence of header but false afterwards to allow multiple
            $replace = true;
            foreach ($values as $value) {
                header("$name: $value", $replace);
                $replace = false;
            }
        }
    }
    $statusCode = $this->getStatusCode();
    header("HTTP/{$this->version} {$statusCode} {$this->statusText}");
    $this->sendCookies();
}