How to use the include keyword in PHP and what to pay attention to

How to use and pay attention to the include keyword in PHP

In PHP programming, the include keyword is a very important statement, used to include the contents of an external file into the current file. This article will introduce the usage and precautions of the include keyword to help readers better understand and use it.

1. Basic usage of the include keyword
The usage of the include keyword is very simple and can be used in the following two ways:

1. Directly reference the file
   include ' filename.php';
   This method is the most common usage scenario, including the contents of the filename.php file into the current file. The included file can be any valid PHP file.
2. Dynamic reference file
   $filename = 'filename.php';
   include $filename;
   This method allows us to store the file name in a variable and then reference it dynamically. This is useful when you need to decide which file to reference based on certain conditions.

No matter which method is used, the include keyword will directly insert the code in the file into the current file when referencing the file, so that the functions, classes, variables, etc. in the referenced file will be in the current file. Visible and available in the file.

2. The difference between include and require
In PHP, we can also use the require keyword to reference files. include and require are basically the same, the only difference is how to handle the case where the referenced file does not exist:

1. include
   include will only generate if the referenced file does not exist A warning and script execution continues.
2. require
   If the referenced file does not exist, require will generate a fatal error and execution of the script will stop.

We can flexibly choose one of them based on specific needs.

3. Notes on include
When using the include keyword, we need to pay attention to the following points:

1. File path
   When referencing a file, pay attention to the file Path settings. Both relative and absolute paths can be used, but relative paths need to be relative to the current working directory.
2. Repeated file references
   When include is used to reference the same file multiple times in the same file, PHP will automatically determine and only reference it once. This is very useful when referencing library files to avoid loading the same code repeatedly.
3. include_once and require_once
   Similar to include and require, include_once and require_once can also be used to reference files. The difference is that they will judge the same file. If it has been referenced, it will no longer be referenced. This avoids errors caused by repeated references.
4. The order of citing files
   When citing multiple files, pay attention to the order of reference. Functions, classes, etc. in the referenced files need to be defined or declared before use.

Summary:
This article introduces the usage and precautions of the include keyword in PHP. By using the include keyword, we can reference the contents of external files into the current file to achieve code reuse and modularization. At the same time, we also learned the difference between include and require, as well as the characteristics of include_once and require_once. I hope that readers will have a deeper understanding of the use of the include keyword through this article and can use it flexibly in actual development.

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