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。
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。
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。
When mounting a file system, you will need to know the correct way to reference it from Linux, and have an empty directory to use as a mount point.
装载一个文件系统时，您需要知道在Linux中对它进行引用的正确方法，还需要有一个空目录作为装载点。
The pathName parameter can be an empty string, or it can reference a file.
参数pathName可以是空字符串，或者引用一个文件。
In a thinly veiled reference to Barack Obama, he said he would fight to make sure "Americans are not deceived by an eloquent, but empty call for change."
他毫不掩饰地针对奥巴马说，要展开斗争，让美国人不被慷慨激昂却空洞无物的变革言词所蒙骗。
Note, however, that copied syntax contains only the method name followed by empty parentheses, so you must remember the parameters or return to content assist or the reference pane.
但是请注意，所复制的语法仅包含方法名称和空括号，因此必须记得参数或者必须返回内容帮助或reference面板。
If there is no entity declaration that matches, an empty text node is attached as the only child of the entity reference node.
如果没有匹配的实体声明，则附加一个空的文本节点作为实体引用节点的唯一子级。
Breaking in took only a few minutes, and he skulked through empty corridors, searching for some sort of directory or reference.
闯入只花数分钟，而且他偷懒过空的走廊，寻找目录或参考的一些类型。
Create an empty game object named PhysicsTest and add script PhysicsTest to it and set both, cube reference and initial velocity of it.
创建一个名为“物理测试”的空游戏对象并添加“物理测试”脚本，并设置它的方块引用和初始速度。
Cannot use an empty string to reference a namespace.
无法使用空字符串引用命名空间。
Forecast of empty container throughput can not only provide reference for dispatch and allocation of empty containers, but also benefit the reasonable planning and task assignment of a port.
空箱吞吐量的预测结果可以作为空箱调运的参考，有利于港口的合理规划和作业安排。
The paper provides a method to use two draught deadweights to estimate the weight of the empty ship. It serves a good reference to the designers.
本文提出利用两个吃水的载重量来估算空船重量的思想和方法，供设计师分析型船资料时作参考。
Strict hierarchical lines within enterprises, the management of humanization reference only empty talks.
企业内部等级界线严格，人性化管理的提法只是一句空话。
Strict hierarchical lines within enterprises, the management of humanization reference only empty talks.
企业内部等级界线严格，人性化管理的提法只是一句空话。
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