引用 - PHP手册笔记

　　PHP中的引用意味着，用不同的变量名访问同一变量内容，类似于Unix的文件名和文件本身(变量名是目录条目，变量内容是文件本身，即用不同的目录条目访问同一文件)，可以看做Unix文件系统中的硬链接。

　　文件系统中的链接有两种，一种被称为硬链接(Hard Link)，另一种被称为符号链接(Symbolic Link)。默认情况下，ln命令产生硬链接。硬连接指通过索引节点来进行的连接。在Linux的文件系统中，保存在磁盘分区中的文件不管是什么类型都给它分配一个编号，称为索引节点号(InodeIndex)。在Linux中，多个文件名指向同一索引节点是存在的。一般这种连接就是硬连接。硬连接的作用是允许一个文件拥有多个有效路径名，这样用户就可以建立硬连接到重要文件，以防止“误删”的功能。其原因如上所述，因为对应该目录的索引节点有一个以上的连接。只删除一个连接并不影响索引节点本身和其它的连接，只有当最后一个连接被删除后，文件的数据块及目录的连接才会被释放。也就是说，文件才会被真正删除。与硬连接相对应，Lnux系统中还存在另一种连接，称为符号连接(Symbilc Link)，也叫软连接。软链接文件有点类似于Windows的快捷方式。它实际上是特殊文件的一种。在符号连接中，文件实际上是一个文本文件，其中包含的有另一文件的位置信息。

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