PHP provides different functions and constants to get the file path or directory. Knowing the differences between them will make your life easier when using include_once() and require_once().

getcwd()

getcwd() function will return the directory path of where the php file is being executed.

From the command line, if you execute from /tmp directory, the output will look like the followings:

/tmp>php -f /var/www/html/path-examples.php 
/tmp

From the browser, if you open http://localhost/path-examples.php, he output will look like the followings:

/var/www/html

__FILE__

__FILE__ will return the path of the PHP file.

From the command line, if you execute from /tmp directory, the output will look like the followings:

/tmp>php -f /var/www/html/path-examples.php 
/var/www/html/path-examples.php

From the browser, if you open http://localhost/path-examples.php, the output will look like the followings:

/var/www/html/path-examples.php

__DIR__

__DIR__ will return the directory path of the PHP file.

From the command line, if you execute from /tmp directory, the output will look like the followings:

/tmp>php -f /var/www/html/path-examples.php 
/var/www/html

From the browser, if you open http://localhost/path-examples.php, the output will look like the followings:

/var/www/html