require VS require_once VS include VS include_once

[sgalmeida]

greetings,

I have this question a quite long time: what are the main diferences between require, require_once, include and include_once?? I've looked all throught the web and the manual but I still don't get it 🙁 Can anyone please tell the differences
and when they should be used?

Thanks

Sérgio

[weekender]

i think that include fetches a file and continues if there's a problem, whereas require MUST HAVE the file and fails without it.

then require_once and include_once will not refetch the file if it has already been required / included

for instance, if you have a file containing code that needs executing at several points, you would not say _once - as this would stop it executing more than once

if you had a file containing functions, you only want that once in each script, to avoid 'function already defined' errors - so you would include or require it only once

[clauslund]

The difference between require and include is that the script will terminate with an error if you use require and the file is not found.

Include example:

<?php
include 'non-existant-file';
echo "hello";
?>

Output:
hello

Require example:

<?php
require 'non-existant-file';
echo "hello";
?>

Output:
Fatal error: Failed opening required 'non-existant-file'

So you will want to use require whenever the content of the included file must be there to continue through your script.

include_once and require_once means that the file will only be included the first time you hit a _once statement.

Example:

include.php

<?php
echo "hello":
?>

script.php

<?php
include_once 'include.php';
if (TRUE) {
    include_once 'include.php';
    echo "Gutentag";
}
else {
     echo "hi";
}
?>

When that script goes through the parser then include.php is only inserted the first time you hit the include_once. So the resulting code actually looks like:

<?php
echo "hello";
if (TRUE) {
    echo "Gutentag";
}
else {
     echo "hi";
}
?>