Let you thoroughly understand include and require in PHP!

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In PHP, there are two ways to include external files, namely include and require. What's the difference between them?

If the file does not exist or an error occurs, require generates an E_COMPILE_ERROR level error and the program stops running. The include only generates a warning and the script will continue to execute.

This is the main difference between them. In other aspects, require is basically the same as include.

• The included file is first searched according to the path given by the parameter. If no directory is given (only the file name), it is searched according to the directory specified by include_path. If the file is not found under include_path, include will finally search in the directory where the calling script file is located and the current working directory.
• If a path is defined, whether it is an absolute path or a relative path to the current directory, include_path will be completely ignored.
• include_path is defined in php.ini
• When a file is included, the code contained in it inherits the variable scope of the line where the include is located. From that point on, any variables available in the calling file at that line are also available in the called file. However, all functions and classes defined in include files have global scope

In addition to ordinary require and include, there are also require_once and include_once. Their functions are:

• If the file has already been included, it will not be included again. As the name of this statement implies, only include once
• can be used when the same file may be included more than once during the execution of the script, and you want to ensure that it is only included once to avoid function redefinition , variable reassignment and other issues

Let’s look at some examples:

// a.php 不存在
include "a.php"; // warning
// require "a.php"; // error

echo 111; // 使用include时111会输出

// file1.php 中只有一行代码echo 'file1';
require_once 'includeandrequire/file1.php'; // file1
require_once 'includeandrequire/file1.php'; // noting

include_once 'includeandrequire/file1.php'; // noting
include_once 'includeandrequire/file1.php'; // noting

require 'includeandrequire/file1.php'; // file1
require 'includeandrequire/file1.php'; // file1

require 'includeandrequire/file1.php'; // file1
require 'includeandrequire/file1.php'; // file1

We can see that when the first _once is loaded successfully, whether it is require_once or include_once, it will This file will not be loaded again. Without _once, the file will be loaded repeatedly.

file2.php

<?php

echo 'file2:' . $a, PHP_EOL;
echo 'file2:' . $b, PHP_EOL;
$b = "file2";

myFile.php

<?php

$a = 'myFile';
$b = 'youFile';
require_once 'includeandrequire/file2.php';
echo $a, PHP_EOL;
echo $b, PHP_EOL;

// 输出结果
// file2:myFile
// file2:youFile
// myFile
// file2

file3.php
<?php

$c = 'file3';

myFile.php
<?php
function test(){
    require_once 'includeandrequire/file3.php';
    echo $c, PHP_EOL; // file3
}
test();
echo $c, PHP_EOL; // empty

The included file can get the variables in the parent file, and the parent file can also get the variables in the included file. However, you need to pay attention to a special case of _once.

function foo(){
    require_once 'includeandrequire/file3.php';
    return $c;
}

for($a=1;$a<=5;$a++){
    echo foo(), PHP_EOL;
}

// file3
// empty
// empty
// empty
// empty

When using _once and loading in a loop, only the content in file3.php will be output for the first time. Why is this? Because the current variable scope is in the method, after the first loading is completed, subsequent files will not be loaded again. At this time, $c is not defined in the next four loops, and $c defaults to a null value.

If a file is loaded using _once in two methods at the same time, will the second method still load it?

function test1(){
    require_once 'includeandrequire/file1.php';
}
function test2(){
    require_once 'includeandrequire/file1.php';
}
test1(); // file1
test2(); // empty

Sorry, only the first method will be loaded successfully, and the second method will not be loaded again.

So, which one is better for us to use in daily development?

• In terms of efficiency, _once needs to verify whether the file has been loaded. The efficiency will be lower, but it is not absolute. It may even be invisible to our naked eyes, so its efficiency problem can be ignored. . And it brings much more benefits than without _once
• Based on the principle of error in advance, it is better to use require_once. Because after adjusting the PHP error level to not display warnings, the warning information of include will not be visible, which will cause unpredictable errors.
• When used in methods, _once should not be used to load files, especially When this file needs to be used in multiple classes or methods, using _once may cause the same file to be unable to be loaded in subsequent methods.
• When using require or include, it is best not to use parentheses, although it is possible. Use, such as include ('xxx.php'); They are expression keywords, not system methods, so you can directly use include 'xxx.php'

include and require files if they exist return, you can use variables to receive the data returned by retun. In addition, they can also load non-PHP files and remote files (remote loading requires making sure allow_url_include in php.ini is On), such as:

file4.php
<?php

return 'file4';

file4.txt
可以吧

myFile.php
<?php
$v = require 'includeandrequire/file4.php';
echo $v, PHP_EOL; // file4

include 'includeandrequire/file4.txt';
// 可以吧

include 'https://www.baidu.com/index.html';
// 百度首页的html代码

Now we have The understanding of include and require is very in-depth. These two ways of loading files are not complicated, but they are also prone to some pitfalls. Especially when _once is used in methods, special attention must be paid to it. Finally, as a small bonus, we encapsulate a method to load all the files in the directory at once:

function include_all_once ($pattern) {
    foreach (glob($pattern) as $file) { 
        require $file;
    }
}

include_all_once('includeandrequire/*');

测试代码：https://github.com/zhangyue0503/dev-blog/blob/master/php/201911/source/%E5%BD%BB%E5%BA%95%E6%90%9E%E6%98%8E%E7%99%BDPHP%E4%B8%AD%E7%9A%84include%E5%92%8Crequire.php

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