


Describe the difference between `include`, `require`, `include_once`, and `require_once` in php.
Jul 11, 2025 am 12:53 AMIn PHP, the main differences between include, require and their \_once versions are in the error handling method and whether the file is loaded repeatedly. 1. include warns when the file cannot be found but the script continues to be executed; 2. require throws a fatal error and terminates the script; 3. include\_once and require\_once respectively ensure that the file is loaded only once during the entire script life cycle to avoid repeated definition errors; 4. The selection is based on whether the file must exist and whether it is possible to introduce it repeatedly.
In PHP, include
, require
, include_once
and require_once
are all used to introduce external files. But there are key differences between them, especially in terms of error handling and whether to load files repeatedly.

include
vs require
The difference between the two is mainly reflected in the different ways of handling errors when the file cannot be found .
-
include
: If the included file does not exist or the path is wrong, PHP will issue a warning (Warning) , but the script will continue to execute. -
require
: If the file cannot be found, PHP will throw a fatal error (Fatal Error) and the entire script will stop running.
For example:

include 'missing-file.php'; // Output a warning, continue to execute echo "continue executing code"; require 'missing-file.php'; // Report an error and terminate the script echo "This line will not be executed";
Therefore, when you need to make sure that a file must exist to continue executing the program (such as configuration files or core library), you should use require
; and when used for optional content, it is more appropriate to include
.
The function of _once
version: prevent duplicate loading
Sometimes you may refer to the same file in multiple places, such as a public function library or configuration file. If no control is added, multiple loading may cause errors such as repeated definitions of functions.

At this time, you need to use the version with _once
:
-
include_once
: Only load the specified file when called for the first time in the current script, and subsequent calls are ignored. -
require_once
: works likerequire
, but it also ensures that the file is loaded only once throughout the script lifecycle.
Examples of usage scenarios:
require_once 'config.php'; // Even if it is called in multiple places, requires_once 'functions.php' will only be loaded once;
This writing can avoid errors like "function already defined" and is especially useful in large projects.
How to choose?
Simply put, you can make a judgment based on the following points:
- Does the file have to exist?
- Yes → Use
require
orrequire_once
- No → Use
include
orinclude_once
- Yes → Use
- Is it possible to introduce it repeatedly?
- Yes → Priority to use
_once
version - No → Normal version
- Yes → Priority to use
Also, in terms of performance, the _once
version will be slightly slower because PHP wants to record which files have been loaded. But in actual development, this difference is almost negligible.
Basically that's it. As long as you understand the two dimensions of "error behavior" and "whether to load repeatedly", you can know when to use which statement.
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