About PHP namespaces

This article introduces the content of PHP namespace, which has certain reference value. Now I share it with everyone. Friends in need can refer to it

What is PHP namespace

PHP Manual: Broadly speaking, namespaces are a way of encapsulating things. This abstract concept can be found in many places. For example, directories are used in operating systems to group related files, and they act as namespaces for the files in the directory.

The role of namespace

1. Name conflicts between user-written code and PHP internal classes/functions/constants or third-party classes/functions/constants.
2. Create an alias (or short) name for a very long identifier name (usually defined to alleviate the first type of problem) to improve the readability of the source code.

Example

I wrote this namespace article because a friend just learned this and asked me what a namespace is. I was wondering how to describe it simply and make it easy to understand. Below I will use a few simple examples to illustrate my own understanding of namespaces.

1. Example 1
First we create two class files
a.php

class Test{
    public function test()
    {
        echo "this is A class.";
    }
}

b .php

class Test{
    public function test()
    {
        echo "this is B class.";
    }
}

Create another index.php file to introduce the above two classes and call the methods in them.
index.php

require_once("a.php");require_once("b.php");

Now run the index.php file, you will find a fatal error: Fatal error: Cannot redeclare class Test in. . . Obviously, the Test class cannot be redeclared because you introduced it twice, and the class names in the two files are the same, which conflicts. At this time, namespace is needed to solve this problem, and it is easy.
2. Example 2
We now slightly modify the two class files.
a.php

namespace a\test;class Test{
    public function test()
    {
        echo "this is A class.";
    }
}

b.php

namespace b\test;class Test{
    public function test()
    {
        echo "this is B class.";
    }
}

The namespace keyword is used to declare the namespace. Now run index.php and find that there are no errors. Modify index.php to perform method call testing
index.php

require_once("a.php");require_once("b.php");$a = new a\test\Test();$a->test();//页面输出：this is A class.

3. Example 3
Now there is another situation. For example, I need to instantiate the Test class in a.php multiple times. So what if it is troublesome to write the complete namespace information every time? What should I do? For example:
index.php

require_once("a.php");require_once("b.php");$a = new a\test\Test();$a_a = new a\test\Test();$a_b = new a\test\Test();$a->test();$a_a->test();//页面输出：this is A class.this is A class.

Although there is no error, you will find it more troublesome. You need to write the full namespace name every time, although no error is reported and you can ctrl c , ctrl v, but not very beautiful (^_^).
You can do this:
index.php

require_once("a.php");require_once("b.php");use a\test\Test;$a = new Test();$a_a = new Test();$a_b = new Test();$a->test();$a_a->test();//页面输出：this is A class.this is A class.

The use keyword is used to introduce a class and uses a namespace to indicate the use of a certain class. . You can directly instantiate the operation later
4. Example 5
Then another question comes, as follows:
index.php

require_once("a.php");require_once("b.php");use a\test\Test;use b\test\Test;$a = new Test();$b = new Test();$a->test();$b->test();

Obviously, another fatal error: Fatal error: Cannot use b\test\Test as Test because the name is already in use in. . . Because although the namespace is used, the two classes have the same name, both are Test. The program does not know that the second Test class is the Test class in b.php. At this time, you use the as keyword
For example:
index.php

require_once("a.php");require_once("b.php");use a\test\Test;use b\test\Test as BTest;$a = new Test();$b = new BTest();$a->test();$b->test();//页面输出：this is A class.this is B class.完美解决

The as keyword defines an alias for the class name, which can effectively prevent conflicts with the same class name
5. Example 6
The following is another situation. I will first give a few code snippets, which are in the Yii2 framework and have nothing to do with the framework. They are just for demonstration. This example can be seen in many places.

if (\Yii::$app->request->isPost) {
            $post = \Yii::$app->request->post();           ...
        }

Obviously a Yii class is used here, but why is there a backslash "\" in front of it? Let's track the Yii class first. Some students will ask how to track it. If you are using PHPstormEditor, just hold down Ctrl and click the class name with the mouse to jump to such class file. For how to use the PHPstorm editor, please check: PhpStorm cracked version and usage tutorial
The following is the Yii class file code snippet:

/**
 * Yii bootstrap file.
 *
 * @link http://www.yiiframework.com/
 * @copyright Copyright (c) 2008 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */require(__DIR__ . '/BaseYii.php');class Yii extends \yii\BaseYii{
}

You will find that the Yii class does not have a namespace. We call this class a global class. If you want to use it, you need to add a backslash "\" in front of the class.
For example, we create a global class file at the same level as a.php: c.php:

class Test{
    public function test()
    {
        echo "this is C class.";
    }
}

Do this in the index.php file to call the test method in c.php

require_once("a.php");
require_once("b.php");
require_once("c.php");
use a\test\Test;use b\test\Test as BTest;
$a = new Test();$b = new BTest();
$c = new \Test();
$a->test();$b->test();
$c->test();
//页面输出：
this is C class.this is A class.this is B class.this is C class.

Note: the usage of keywords such as namespace, use, as and the use of global classes.

What is a PHP namespace

PHP Manual: Broadly speaking, a namespace is a way of encapsulating things. This abstract concept can be found in many places. For example, directories are used in operating systems to group related files, and they act as namespaces for the files in the directory.

The role of namespace

1. Name conflicts between user-written code and PHP internal classes/functions/constants or third-party classes/functions/constants.
2. Create an alias (or short) name for a very long identifier name (usually defined to alleviate the first type of problem) to improve the readability of the source code.

示例

写这个命名空间的文章是因为一个朋友刚学这个来问我命名空间到底是个什么东西。我在想怎么简单的描述出来并且很容易理解。下面我就以几个简单的小例子来说明一下我自己对命名空间的理解。

1. 例一
首先我们先建立两个类文件
a.php

class Test{
    public function test()
    {
        echo "this is A class.";
    }
}

b.php

class Test{
    public function test()
    {
        echo "this is B class.";
    }
}

再建立一个index.php文件，来引入以上两个类并调用其中的方法。
index.php

require_once("a.php");require_once("b.php");

现在运行index.php文件，你会发现有一个致命错误： Fatal error: Cannot redeclare class Test in。。。很显然，无法重新声明Test类，因为你引入了两次，而且两个文件中的类名称相同，冲突了。这个时候就需要命名空间来解决这个问题，并且很容易。
2. 例二
我们现在把两个类文件稍作修改。
a.php

namespace a\test;class Test{
    public function test()
    {
        echo "this is A class.";
    }
}

b.php

namespace b\test;class Test{
    public function test()
    {
        echo "this is B class.";
    }
}

namespace关键字是用来声明命名空间的。现在运行index.php发现没有错误，修改index.php进行方法调用测试
index.php

require_once("a.php");require_once("b.php");$a = new a\test\Test();$a->test();//页面输出：this is A class.

3. 例三
现在有另外一种情况，比如我需要实例化a.php中的Test类多次，那么每次我们都需要命名空间信息完整写的话比较麻烦怎么办呢？比如：
index.php

require_once("a.php");
require_once("b.php");
$a = new a\test\Test();
$a_a = new a\test\Test();
$a_b = new a\test\Test();
$a->test();
$a_a->test();
//页面输出：
this is A class.this is A class.

虽然也没有错误，但是你会发现比较麻烦，每次都需要全写命名空间名称，虽然不报错并且可以ctrl+c，ctrl+v，但是不太美观（^_^）。
你可以这样做：
index.php

require_once("a.php");
require_once("b.php");
use a\test\Test;
$a = new Test();
$a_a = new Test();
$a_b = new Test();
$a->test();
$a_a->test();
//页面输出：
this is A class.this is A class.

use关键字是用来引入类，用命名空间的方式表示使用了某个类。后面就可以直接实例化操作
4. 例五
接下来另一个问题又来了，如下：
index.php

require_once("a.php");require_once("b.php");use a\test\Test;use b\test\Test;$a = new Test();$b = new Test();$a->test();$b->test();

很明显，又一个致命错误：Fatal error: Cannot use b\test\Test as Test because the name is already in use in 。。。因为虽然使用了命名空间，但是两个类名称相同，都是Test，程序不知道第二个Test类是b.php中的Test类，这时候你就用到了as关键字
如：
index.php

require_once("a.php");
require_once("b.php");
use a\test\Test;
use b\test\Test as BTest;
$a = new Test();
$b = new BTest();
$a->test();$b->test();
//页面输出：
this is A class.this is B class.完美解决

as关键字是对类名称定义别名，可以有效防止类名称相同冲突
5. 例六
下面是另一种情况，我先给出几个代码片段，是Yii2框架中的，和框架无关，只是为了演示使用，很多地方都能见到此例。

if (\Yii::$app->request->isPost) {
            $post = \Yii::$app->request->post();           ...
        }

很显然这里使用了一个Yii类，但是为什么前面又一个反斜杠”\”，我们先追踪一下Yii类，有的同学会问怎么追踪呢，如果你使用的是PHPstorm编辑器，直接按住Ctrl，鼠标点击类名就会跳到此类类文件中，关于怎么使用PHPstorm编辑器，请查看：PhpStorm破解版及使用教程
下面是Yii类文件代码段：

/**
 * Yii bootstrap file.
 *
 * @link http://www.yiiframework.com/
 * @copyright Copyright (c) 2008 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */require(__DIR__ . '/BaseYii.php');class Yii extends \yii\BaseYii{
}

你会发现该Yii类并没有命名空间，我们将这种类叫做全局类，如果要使用需要类前面加入反斜杠”\”
比如我们在a.php同级再建立一个全局类文件：c.php：

class Test{
    public function test()
    {
        echo "this is C class.";
    }
}

在index.php文件中这样做即可调用c.php中的test方法

require_once("a.php");
require_once("b.php");
require_once("c.php");
use a\test\Test;
use b\test\Test as BTest;
$a = new Test();
$b = new BTest();
$c = new \Test();
$a->test();
$b->test();
$c->test();
//页面输出：
this is C class.this is A class.this is B class.this is C class.

注意：namespace，use，as等关键字用法以及全局类的使用。

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