Including external files in PHP

To use your code across many PHP scripts, PHP provides four expressions to load an external script:

include
include_once
require
require_once

include

If you want to reuse a function in more than one script, you can store it in a file and then use one of the above-mentioned expressions to include that function in your desired scripts. For example, you can create a file called functions.php and put the function/code in the file:

<?php
 # functions.php
 function doublevalue($var) {
  return $var * 2;
 }

In a script, you can then use the include statement to provide access to the function doublevalue():

<?php
 # testing.php
 include "functions.php";
 echo doublevalue(10); # Prints: 20

The script works as before, but the function doublevalue() can now be reused across several scripts by including functions.php file.

The include statement is treated in the same way as other statements. For example, you can conditionally include different files using the following code fragment:

<?php
 $post = $_POST['submit'] ?? "";
 if ($post == "") {
  include "form.php";
 }
 else {
  include "verify.php";
 }

Any PHP code in an include file must start with the PHP start tag. The PHP script engine treats the contents of include files as HTML unless script tags are used.

Include_once

Be careful when using the include statement. Including the same file twice or declaring a function in the script that is already in an include file causes PHP to complain about the function being redefined, consider the following example:

<?php
 # testing.php
 //Including twice functions.php
 include "functions.php";
 include "functions.php";
 echo doublevalue(10);

 /* Fatal error: Cannot redeclare doublevalue()
 (previously declared in ... functions.php:2 */

It is recommended to use include_once and require_once because these directives first check whether the file has already been loaded before either will load a given script, consider the same example using include_once directive:

<?php
 # testing.php
 //Including twice functions.php
 include_once "functions.php";
 include_once "functions.php";
 echo doublevalue(10); # Prints: 20

The above code will not print any error.

require

The require is identical to include except upon failure it will also produce a fatal level error and halt the script whereas include only emits a warning which allows the script to continue.

<?php 
 include 'fakefile.php';
 echo 'Hello world';
 # Warning: include(fakefile.php): Failed to open stream
 # Hello world
 
 require 'fakefile.php';
 echo 'Hello world again'; # not prints
 # Fatal error: Uncaught Error: Failed opening required 'fakefile.php'

require_once

The require_once expression is identical to include_once except upon failure it will produce a fatal level error and stop the script whereas include_once only emits a warning which allows the script to continue.

User-defined functions: