What is the difference between include, require, include_once, require_once?

In PHP, the difference between include, require, include_once, require_once is: 1) include generates a warning and continues to execute, 2) require generates a fatal error and stops execution, 3) include_once and require_once prevent repeated inclusions. The choice of these functions depends on the importance of the file and whether it is necessary to prevent duplicate inclusion. Rational use can improve the readability and maintainability of the code.

introduction

In PHP programming, include , require , include_once , and require_once are the file inclusion tools we often use. They seem similar, but in fact there are subtle but important differences in usage scenarios and handling methods. Today we will explore the differences between these functions in depth to help you better understand and apply them.

Through this article, you will learn how to choose the right file containing functions in different situations, avoid common errors, and improve the readability and maintainability of your code.

Review of basic knowledge

In PHP, file inclusion is a way to insert the contents of an external file into the current script. This not only improves the reusability of the code, but also makes the project structure clearer. include and require are the most basic file include functions, while include_once and require_once add functions to prevent duplicate inclusion on their basis.

Core concept or function analysis

Definition and function

• include : This function will include the specified file into the current script. If the included file does not exist, include will generate a warning (E_WARNING), but the script will continue to execute.
• require : Similar to include , but if the file does not exist, require will generate a fatal error (E_COMPILE_ERROR) and the script will stop executing.
• include_once : This function is similar to include , but it checks whether the file has been included, and if so, it will not be included again.
• require_once : Similar to require , but it also checks if the file has been included, and if so, it will not be included again.

How it works

When you use these functions, PHP tries to read and execute the contents of the specified file. If the file does not exist, include and require are handled differently:

• include will continue to execute the script, which may be useful in some cases, such as during debugging or development, you may want the script to continue to run in order to see more error messages.
• require will immediately stop the execution of the script because it considers the included files necessary for the script to run.

include_once and require_once will check an internal include list before including the file to ensure that the file will not be included repeatedly. This is useful when including function or class definitions that can cause conflicts.

Example of usage

Basic usage

Suppose we have a configuration file called config.php , which we want to use in different scripts:

 // Use include
include 'config.php';

// Use require
require 'config.php';

// Use include_once
include_once 'config.php';

// Use require_once
require_once 'config.php';

In these examples, if config.php does not exist, include will generate a warning, and require will cause the script to stop. include_once and require_once ensure that config.php is included only once.

Advanced Usage

Suppose we have a file functions.php with multiple functions that we want to use in different scripts, but don't want to repeatedly define:

 // include_once 'functions.php' in script1.php;
myFunction1();

// include_once 'functions.php' in script2.php;
myFunction2();

In this example, include_once ensures that functions.php is not included repeatedly, thus avoiding the error of function redefinition.

Common Errors and Debugging Tips

• File path error : Make sure that the file path you are using is correct, both relative and absolute paths can be used, but be careful about the current working directory.
• Repeat Inclusion : Use include_once or require_once to avoid duplicate include files, especially when including files that define functions or classes.
• Performance issues : Too many file inclusions may affect the performance of the script, organize the code structure reasonably, and reduce unnecessary inclusions.

Performance optimization and best practices

In practical applications, selecting the right file containing function can significantly improve the readability and maintainability of the code:

• Use require to include required files : For scripts to run required files, use require or require_once to ensure that the script stops executing immediately when the file does not exist.
• Use include to include to include : For optional files, use include or include_once so that the script can continue to run when the file does not exist.
• Avoid overuse of file inclusion : Too many file inclusions will increase the complexity and loading time of the script, organize the code structure reasonably, and reduce unnecessary inclusions.
• Code readability : When using file inclusion, add appropriate comments to explain why this file is used and what it contains.

By understanding and correctly using include , require , include_once , require_once functions, you can better manage your PHP projects and improve the quality and efficiency of your code.

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