1-include one file includes more times but include_once one file includes only one times.
2-The include_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include() statement, with the only difference being that if the code from a file has already been included, it will not be included again.
3-If file not found then show warning error but continue execution of the script.
I have three files
When open file header.php you will get error because config.php already includes functions.php