Can someone explain deeply in include and require in php?

Question

I know include and require is used for inserting php in other files like html files. But what I can undertand is the timing of the use of both. Can anyone explain deeply into the two of them? I find some information online but they're all not well explained I think, I mean not clear enough.

Answer 1

In the PHP documentation about required they say this:

require is identical to include except upon failure it will also produce a fatal E_COMPILE_ERROR level error. In other words, it will halt the script whereas include only emits a warning (E_WARNING) which allows the script to continue.

and for include the say this

The include statement includes and evaluates the specified file. read more

Explicit example

This line using include is going to include the Php file because the file exists.

<? include("file1.php"); //This file exists ?>

This line using include is going to show a warning in your interface because the file does not exist. Showing up this message on your interface depends on the level of error you have put in your php.ini

<? include("filenotexist.php"); //This file NO exists ?>

Result

Warning: include(filenotexist.php) [function.include]: failed to open stream: No such file or directory in ****** on line 6
Warning: include() [function.include]: Failed opening 'filenotexist.php' for inclusion (include_path='.;C:\php\pear') in **** on line 6

This line using required is going to do the same as the include, it's going to include the php file because the file exists.

<? require("file1.php"); //This file exist ?>

This line using required, so you get a warning and a fatal error because the file does not exists. This fatal error is the main difference you are looking for.

<? require("filenoexists1.php"); //This file NO exist ?> 

Result

Warning: require(filenoexists1.php) [function.require]: failed to open stream: No such file or directory in **** on line 6
Fatal error: require() [function.require]: Failed opening required 'filenoexists1.php' (include_path='.;C:\php\pear') in *** on line 6

And there is even another interesting function called required_once. If you required several times the same file

<? require("hola.php"); //This file exist ?> 
<? require("hola.php"); //This file exist ?> 
<? require("hola.php"); //This file exist ?> 
<? require("hola.php"); //This file exist ?> 
<? require("hola.php"); //This file exist ?> 

The result is 5 times hola. It makes sense because you required the file 6 times.

hola hola hola hola hola

Required_once even if you include it 5 times, Php will only include it once.

<? require_once("hola.php"); //This file exist ?> 
<? require_once("hola.php"); //This file exist ?> 
<? require_once("hola.php"); //This file exist ?> 
<? require_once("hola.php"); //This file exist ?> 
<? require_once("hola.php"); //This file exist ?> 

the result is

hola

Hope it helps Daniel

Answer 2

PHP's include, require and, to complete the list, include_once and require_once, are statements that allow one PHP file to fetch another file (in most cases another PHP file), and insert and evaluate the fetched file at the position of the statement.

These statements are not, as you wrote, for "inserting php in other files like html files", instead they are the concept that allows a PHP application to have its codebase structured into separate files. Code fetching other code fetching other code and so on.

The difference between the mentioned statements is basically:

include 'a/file/that/does/not/exist.php'; //emits a warning, but doesn't stop the script 

require 'a/file/that/does/not/exist.php'; //throws a fatal error and stops script execution

... and using require_once and include_once, you can prevent 'double-includes', which could cause problems like function redefinitions