Including and Requiring Files in PHP: Syntax Variations
Jul 16, 2025 am 03:18 AMThe difference between include and require in PHP is in the error handling method: if include fails, warning will be issued and execution will be continued, and if require fails, fatal error will be triggered and script will be terminated; if _once is used to avoid duplicate loading; if key files are used to require or require_once, non-critical files are used to include or include_once; class libraries are recommended to use _once; paths are recommended to use __DIR__ stitching.
In PHP, including and introducing files are very common operations during development. However, different syntaxes will bring different effects in actual use, especially in error handling and loading logic. If you encounter file path problems, duplicate loading problems during development, or are not sure whether to use include
or require
, this article can help you clarify your ideas.

The difference between include and require: the error handling method is different
This is the most core difference. When you use:
-
include 'file.php';
: If the file cannot be found or fails to load, PHP will issue a warning , but the script will continue to execute. -
require 'file.php';
: If the file cannot be found or fails to load, PHP will issue a fatal error and the script will terminate directly.
For example, if you introduce a non-critical statistical script at the bottom of the page, it is more appropriate to use include
; while a critical dependency like a database connection file should be used to require
, because once the load fails, it is meaningless to continue execution.

_once version: Avoid duplicate loading
If you are worried that a file will be loaded multiple times, you can use:
-
include_once
-
require_once
These two statements will check whether the file has been loaded and will not be executed again if it has been loaded. This is very useful for preventing duplicate definitions of functions, duplicate declarations of classes, etc.

For example, you have a structure like this:
// a.php function foo() { echo "Hello"; } // b.php include 'a.php'; // index.php include 'a.php'; include 'b.php';
An error will be reported at this time because a.php
has been loaded twice. Change include_once
or require_once
to avoid this problem.
Scenario suggestions: Choose the appropriate syntax according to your needs
- Key files (such as configuration, database connection) : Use
require
orrequire_once
to ensure that the file exists and loads successfully. - Non-critical files (such as statistical code, module components) : Use
include
orinclude_once
, which will not affect the main process even if the load fails. - Class library or function library file : It is recommended to use the
_once
form to prevent errors caused by repeated loading.
In addition, if you use an automatic loading mechanism (such as Composer's autoloader), you don't need to manually write include
or require
, and automatic loading will help you handle these logic.
File path writing: relative path, absolute path and magic constant
Path writing will also affect loading results. Common writing methods include:
- Relative path:
include 'config.php';
orinclude '../lib/functions.php';
- Absolute path:
include '/var/www/html/project/config.php';
- Use magic constants:
include __DIR__ . '/config.php';
It is recommended to use __DIR__
stitching paths, which can avoid path errors when calling relative paths at different levels.
for example:
// Assume that the current file is located in /project/includes/ include __DIR__ . '/../config.php';
In this way, no matter where the current file is called, the path can be parsed correctly.
Basically that's it. Understanding the differences between different syntaxes can help you write more stable and robust PHP programs.
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