Dirname (_ file _) in PHP

In PHP, dirname (_ file _) is a very effective method to use dirname (_ FILE _) when configuring the file path, however, because the path of _ FILE _ is the complete path of the FILE where the current code is located (rather than the FILE where the url is located), the configuration FILE must be defined under the root directory to define the root address of the website, however, the following method can solve the storage problem of configuration files.
Dirname (_ FILE __));/*
Assume that _ FILE _ is/home/web/config. php.
The above method output is/home/web
*/
Dirname (_ FILE _); the directory name at the top of the FILE is obtained.
Dirname (_ FILE _); the directory name of the FILE's layer is obtained.