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What is the difference between include, include_once in PHP

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.

Example-

I have three files
1-header.php
2-config.php
3-functions.php

functions.php

function test(){
echo "Hello";
}

config.php

include_once('functions.php');
test();

header.php

include_once('functions.php');
include('config.php');

When open file header.php you will get error because config.php already includes functions.php