File location:
Absolute path: starting from the root directory of the disk (local absolute path) C: / path / PHP file
Start (absolute network path) from the site with directories
/: Host name of the site in relation to the corresponding path
Relative path: the path from the current directory to open the file is located
Or ./: Indicates the current folder
../:Parent directory.
Nested include files
Nested file contains: a file contains another file, but also contains the file contains another file.
index.php include introducing a.php (a file itself contains include b.php)