PHP Control Structure Part -5: PHP File Inclusion Functions

What is File Inclusion Functions in PHP?

In PHP, Include, require, include_once, and require_once functions is used for add file contents to the current file. However, if you want to include any remote PHP server file at your own server, then you must give permission to the allow_url_include on php.ini file. However, allow_url_include is harmful on the website’s security.

How to use include, include_once and require, require_once functions on PHP?

Include, include_once and require, require_once functions are as like echo and print language construct, so you can use these functions as well as with parentheses or without parentheses. Notice the following examples:

<?php
include("a.php"); //with parentheses 
//Or
include "a.php"; //without parentheses 
?>

In the line where you want to include the file, the file will work in the following lines. Now let us see how include function works in the following example:

<?php
//vars.php
 
$color = 'green';
$fruit = 'apple';
 
?>
 
 
<?php
//test.php
 
echo "$color $fruit"; // will show warning
 
include 'vars.php';
 
echo "$color $fruit"; // Result: green apple
 
?>

include function also can use in inside user defined function. In this case, the user defined function will be included only after the call. Notice the following examples:

<?php
//vars.php
 
$color = 'green';
$fruit = 'apple';
 
?>
<?php
//foo.php
function foo(){
    include 'vars.php';
    echo "A $color $fruit";
}
foo();                    // A green apple
?>

We also can include the file via http request in PHP. See the example below:

include 'http://www.example.com/file.php?foo=1&bar=2';

What is the difference between require and require_once, include and include_once function in PHP?

There are many differences between require and require_once, include and include_once function in PHP. Notice the following table:

requires and include	VS	require_once and include_once
include()It includes a specified file. It will produce a warning if it fail to find the file and execute the remaining scripts require():It includes a specified file. It will produce a fatal error if it fail to find the file and stops the execution		include_once()It includes a specified file. a file has already been included, it will not be included again. It will produce a a warning if it fail to find the file and execute the remaining scripts. require_once(): It includes a specified file. a file has already been included, it will not be included again. It will produce a a fatal error if it fail to find the file and stops the execution.