You can test for them explicitly or rely on PHP to return a default empty value when you reference them.

您可以显式地对它们进行检测，或在您引用它们时让 PHP返回缺省的空值。

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To add a reference manually, you can edit the empty reference using the instructions in the previous section.

手动添加参考文献，可以参考前一节进行编辑空白参考文献。

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When the digital fingerprint of the referenced resource (the XFDL document) is calculated, the DigestValue element of the Reference is empty.

当所引用的资源（XFDL 文档）的数字指纹被计算后，Reference 的 DigestValue 元素将为空。

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