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The include_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include() statement, with the only difference being that if the code from a file has already been included, it will not be included again. As the name suggests, it will be included just once.

include_once() may be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, so in this case it may help avoid problems such as function redefinitions, variable value reassignments, etc.

See the include() documentation for information about how this function works.


With PHP 4, _once functionality differs with case-insensitive operating systems (like Windows) so for example:

Example #1 include_once() with a case insensitive OS in PHP 4

include_once "a.php"// this will include a.php
include_once "A.php"// this will include a.php again! (PHP 4 only)

This behaviour changed in PHP 5, so for example with Windows the path is normalized first so that C:\PROGRA~1\A.php is realized the same as C:\Program Files\a.php and the file is included just once.

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User Contributed Notes 7 notes

roach dot scott+spam at googlemail dot com
8 years ago
If you include a file that does not exist with include_once, the return result will be false.

If you try to include that same file again with include_once the return value will be true.

(include_once 'fakefile.ext'); // bool(false)
var_dump(include_once 'fakefile.ext'); // bool(true)

This is because according to php the file was already included once (even though it does not exist).
Greg McCarthy
29 days ago
In response to what a user wrote 8 years ago regarding include_once being ran two times in a row on a non-existent file:

Perhaps 8 years ago that was the case, however I have tested in PHP 5.6, and I get this:

$result = include_once 'fakefile.php';  // $result = false
$result = include_once 'fakefile.php'   // $result is still false
xcl_rockman at qq dot com
1 year ago
return array("test">1);

$config = include_once("config.php");

$config = include_once("config.php");

output will be

webmaster AT domaene - kempten DOT de
10 years ago
Since I like to reuse a lot of code it came handy to me to begin some sort of library that I stored in a subdir
e.g. "lib"

The only thing that bothered me for some time was that although everything worked all IDEs reported during editing
these useless warnings "file not found" when library files included other library files, since my path were given all relative to the corresponding document-root.

Here is a short workaround that makes that gone:

// Change to your path

if(strpos(__FILE__,'/lib/') != FALSE){
include_once (
// ... or any other include[_once] / require[_once]

just adjust the path and it will be fine - also for your IDE.

1083706899 at qq dot com
1 year ago
require_once() can check the file if once include ,or the file is wrong will tell a error and quit the script.
flobee at gmail dot com
11 years ago
i already had a discussion with several people about "not shown errors"
error reporting and all others in php.ini set to: "show errors" to find problems:
the answer i finally found:
if you have an "@include..." instead of "include..." or "require..('somthing') in any place in your code
all following errors are not shown too!!!

so, this is actually a bad idea when developing because paser errors will be droped too:
if(!@include_once('./somthing') ) {
'can not include';

if(!@file_exists('./somthing') ) {
'can not include';
} else {
emanuele at rogledi dot com
8 years ago
For include_once a file in every paths of application we can do simply this

include_once($_SERVER["DOCUMENT_ROOT"] . "mypath/my2ndpath/myfile.php");
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