_ FILE _ constant usage in php

The _ FILE _ constant in php returns the complete path of the FILE and the FILE name. dirname (_ FILE ___) function returns the path where the script is located. The _ FILE _ constant in php returns the complete FILE path and FILE name.
The dirname (_ FILE ___) function returns the path where the script is located.
For example, file B. php contains the following content:
$ Basedir = dirname (_ FILE __);
?>
If B. php is referenced by the. php file require or include in another directory.
Variable? The content of $ basedir is still the path of the folder where B. php is located.
Instead of the directory where the. php file is located.
Dirname (_ FILE _) usually returns a directory structure from the current directory of the FILE to the root directory of the system.
The current file name is not returned.
Dirname (_ FILE _) may also return a. (current directory)
[The reason is that the B. php file is in the default WEB directory where http. conf or PHP is used to configure the development environment.
For example, WEB_ROOT is: "C:/root/www/".]
B. php file path: "C:/root/www/B. php ".
Usage tips,
If you repeat the command, you can raise the directory to a higher level:
For example :? $ D = dirname (_ FILE __));
In fact, a directory is used as a parameter to dirname (), because the last Directory returned by dirname () does not contain \ or/
Therefore, we can think that dirname () treats the bottom directory as a file name.
The upper-level Directory of the current directory.
Include the files to obtain the upper-level Directory
Include (dirname (_ FILE _). '/../filename. php ');