XXVII. File Alteration Monitor Functions
FAM monitors files and directories, notifying interested applications of changes.
A PHP script may specify a list of files for FAM to monitor using the functions provided by this extension.
The FAM process is started when the first connection from any application to it is opened. It exits after all connections to it have been closed.
由于这些常量是由该扩展模块定义的，因此只有在该扩展模块被编译到 PHP 中，或者在运行时被动态加载后，这些常量才有效。