这实际上与Zend Framework无关，但是你需要为DomPDF提供正确的路径来加载外部文件。

This has in fact nothing to do with Zend Framework, but you need to supply DomPDF the right path to load the "external" files from.

$dompdf = new DOMPDF();
$dompdf->set_base_path(realpath(APPLICATION_PATH . '/path/to/css/'));
$dompdf->load_html($html);
$dompdf->render();

另请参阅手动此功能的DomPDF。

See also the manual of DomPDF for this feature.