The difference between include, include_once, require, and require_once

The first thing to say is that the core functions of include, include_once, require, and require_once are all the same. They are equivalent to directly copying the code of the target web page.

Basically, if you don’t consider performance, you can achieve basic results by using include, include_once, require, and require_once. Basically, use it however you like.

The minor differences are as follows:

For example, there is a simple print statement in 1.php:

<?php echo "1" ?>

The result of running the following program:

<?php include &#39;1.php&#39;;
require &#39;1.php&#39;;

include_once &#39;1.php&#39;;
require_once &#39;1.php&#39;;
?>

is 2 1s instead of 4 1s, because include and require both import the specified file, _once means importing it only once, that is, the files that have been introduced before will not be imported again.

If written as:

<?php include_once &#39;1.php&#39;;
require_once &#39;1.php&#39;;
include &#39;1.php&#39;;
require &#39;1.php&#39;;
?>

the result will be 4 ones.

It is worth noting that although _once means that what has been introduced before will no longer be introduced, this statement will not check whether the code in the imported web page is the same. For example, there are two web pages with exactly the same code, untitled.html and untitled1.html. All are:

<meta http-equiv="Content-Type" c charset='utf-8"'>
<title>无标题文档</title>

If there is the following sentence:

<?php include_once &#39;untitled.html&#39;;
include_once &#39;untitled1.html&#39;;
?>

The result is like this:

See the same part appearing twice. _once just uses the file name to determine whether it has been introduced before or not.

So _once cannot prevent the phenomenon of the same resource being introduced multiple times with different names.

In addition to the different ways of handling imported files, the biggest difference between include and require is that include generates a warning when introducing a file that does not exist and the script will continue to execute, while require will cause a fatal error and the script will stop executing. .

For example, if a.php does not exist, the following code will still output b:

<?php include &#39;a.php&#39;;
echo &#39;b&#39;;
?>

but:

<?php require &#39;a.php&#39;;
echo &#39;b&#39;;
?>

will not.

There are also the following differences:

include() is a conditional inclusion function, while require() is an unconditional inclusion function

if(FALSE){ 
    include 'file.php'; //file.php不会被引入 
} 

if(FALSE){ 
    require 'file.php'; //file.php将会被引入
｝

include has a return value, while require does not

$retVal = include(’somefile.php’);
if(!empty($retVal)){
  echo “文件包含成功”;
}else{
  echo “文件包含失败”;
}

is okay.

The files that need to be referenced during include() execution must be read and evaluated every time. The files that need to be referenced when require() is executed are only processed once. In fact, the content of the file that needs to be referenced during execution replaces the require() statement. It can be seen that if there is code that contains one of these instructions and code that may be executed multiple times, it is more efficient to use require(). If a different file is read each time the code is executed or there is an iteration through a set of files. To loop, use include().

require is usually used. This function is usually placed at the front of the PHP program. Before the PHP program is executed, it will first read in the file specified by require and make it a part of the PHP program web page. Commonly used functions can also be introduced into web pages in this way. The include method is usually used. This function is usually placed in the processing part of the process control. The PHP program webpage only reads the include file when it reads it. In this way, the process of program execution can be simplified.

In addition, regarding the issue of whether to add parentheses after include and require, theoretically: there is no difference in the execution results whether there are parentheses after include or require, but adding parentheses is less efficient, so if there is no parentheses later, then do not add parentheses. .

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The above introduces the differences between include, include_once, require, and require_once, including the relevant content. I hope it will be helpful to friends who are interested in PHP tutorials.