Difference Between include () and include_once () in php

The major difference between include () vs include_once() in php is,

The both functions working performance is same
The include_once() is used to include any user defined which was written by you functions.php script. As the name suggests, it will be included just once.

In details now understand by below example

Let's i have 3 files
FUNCTIONS.PHP
GLOBALS.PHP
HEADER.PHP

FUNCTIONS.PHP
<?php
  function funname(){
         echo "Hello World..!";
  }
?>

GLOBALS.PHP
<?php
   include "FUNCTIONS.PHP";
   funname();
?>

HEADER.PHP
<?php
  include "FUNCTIONS.PHP";
  include "GLOBALS.PHP";
  funname();
?>

If you try to access the HEADER.PHP you will get an error, this is about you already declared function in GLOBALS.PHP, and again included in HEADER.PHP - which means you included same FUNCTIONS.PHP by 2 times.

So the best practice rewrite the code as below

HEADER.PHP
<?php
  include_once ('FUNCTIONS.PHP');
  include "GLOBALS.PHP';
?>
now if you try to access the HEADER.PHP, you will never get error.

Note: In short remember when we have functions in the script at this time better to use include_once() else you can use include()

I hope you understood the difference