They hold only one piece of data : a string a number the undefined value a reference to another variable .

它只保存一段数据：一个字符串、一个数字、 未 定义的值以及对另一个 变量的引用。

Instead of checking for a default compound value you can also catch the access to an undefined variable . E.g.

不用再检查默认的复合值，您也可以捕获对未 定义 变量的访问。

If you further set xdebug.dump_undefined to On and a named superglobal variable isn 't set the variable is still printed with the value undefined .

如果进一步将xdebug.dumpundefined设为On并且不设定指定的超全局 变量，则仍用值 undefined打印变量。

In the snippet above it looks as though the script is using an undefined variable named alertConfigService but this is not the case .

在上面的代码片段中，好像脚本使用了 未 定义 的 变量“ alertConfigService”，但事实并非如此。