Detailed explanation of the difference between include and require in PHP, includerequire

1. Summary

The performance of the require() statement is similar to include(), both including and running the specified file. The difference is that for the include() statement, the file is read and evaluated each time it is executed; while for require(), the file is only processed once (in fact, the file content replaces the require() statement ). This means that if the code is likely to be executed multiple times, it is more efficient to use require(). On the other hand, if you are reading a different file each time the code is executed, or if you have a loop that iterates through a set of files, use the include() statement.

The method of using require is as follows: require("myfile.php"). This statement is usually placed at the front of the PHP script program. Before the PHP program is executed, it will first read the file introduced by the require() statement, making it a part of the PHP script file. The use of include is the same as require, such as include("myfile.php"), and this statement is generally placed in the processing section of the flow control. The PHP script file only reads the files it contains when it reads the include() statement. In this way, the process of program execution can be simplified.

include is loaded when used
require is loaded at the beginning
The _once suffix indicates that the loaded one is not loaded.

PHP system has a pseudo-compilation process when loading a PHP program, which can make the program run faster. But the documentation for include is still interpreted. If there is an error in the include file, the main program will continue to execute. If there is an error in the require file, the main program will also stop. Therefore, if errors in the included files have little impact on the system (such as interface files), use include, otherwise use require.

The require() and include() statements are language structures, not real functions. They can be like other language structures in PHP. For example, echo() can use echo("ab") form, or echo "abc" "Outputs the string abc in the form. The require() and include() statements can also add parameters directly without parentheses.

The include_once() and require_once() statements also include running the specified file during script execution. This behavior is similar to the include() statement and require(), and its usage is the same. The only difference is that if the code in the file is already included, it will not be included again. These two statements should be used when the same file may be included more than once during script execution to ensure that it is only included once to avoid problems such as function redefinition and variable reassignment.

2. Details

2.1 Error reporting

When include introduces a file, if an error is encountered, a prompt will be given and the code below will continue to run.

When require imports a file, if an error is encountered, a prompt will be given and the code below will stop running.

Using an example, write two PHP files named test-include.php and test-require.php. Note that there should not be a file named test-nothing.php in the same directory.

<p>test-include.php</p>
<p><?php</p>
<p>include 'test-nothing.php';</p>
<p>echo 'abc';</p>
<p>?></p>

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<p>test-require.php</p>
<p><?php</p>
<p>require 'test-nothing.php';</p>
<p>echo 'abc';</p>
<p>?></p>

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Browse http://localhost/test-include.php. Because the test-nothing.php file was not found, we saw the error message. At the same time, abc is displayed below the error message. What you see may be similar to the one below. Situation:

<p>Warning: include(test-nothing.php) [function.include]: failed to open stream: No such file or directory in D:\www\test-include.php on line 2</p>
<p>Warning: include() [function.include]: Failed opening 'test-nothing.php' for inclusion (include_path='.;C:\php5\pear') in D:\www\test-include.php on line 2</p>
<p>abc</p>

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Browse http://localhost/test-require.php. Because the test-nothing.php file was not found, we saw the error message. However, abc is not displayed below the error message. What you see may be similar to the one below. Situation:

<p>Warning: require(test-nothing.php) [function.require]: failed to open stream: No such file or directory in D:\www\test-require.php on line 2</p>
<p>Fatal error: require() [function.require]: Failed opening required 'test-nothing' (include_path='.;C:\php5\pear') in D:\www\test-require.php on line 2 </p>

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2.2 Conditional Reference

The functions of include() and require() are the same, but there are some differences in usage. include() is a conditional inclusion function, while require() is an unconditional inclusion function.

For example, in the following example, if the variable $somg is true, the file somefile.php will be included:

<p>if($some){</p>
<p>　　include 'somefile.php';</p>
<p>}</p>

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But no matter what value $some takes, the following code will include the file somefile.php into the file:

<p>if($something){</p>
<p>　　require 'somefile.php';</p>
<p>}</p>

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The following example fully illustrates the difference between these two functions:

<p>$i = 1;</p>
<p>while ($i < 3) {</p>
<p>　　require "somefile.$i.php";</p>
<p>　　$i++;</p>
<p>}</p>

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It can be seen from the above code that the program will include the same file every time it loops. Obviously this is not what we want. It can be seen that this code hopes to include the same file every time it loops. , include different files. If you want to complete this function, you can only use the function include():

<p>$i = 1;</p>
<p>while ($i < 3) {</p>
<p>　　include "somefile.$i.php";</p>
<p>　　$i++;</p>
<p>}</p>

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2.3 File Reference Method

The files that need to be referenced when include() is executed must be read and evaluated each time, and 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 iteration through a set of files For loops, use include(). You can set a variable for the file name you want to include. This variable is used when the parameter is include().