Usage of PHP command space namespace and use (example)

This article mainly introduces the practical summary of the usage of PHP command space namespace and use. Friends who need it can refer to it.

One of the clearest purposes of namespace is to solve the problem of duplicate names. Two names are not allowed in PHP. If two functions or classes have the same name, a fatal error will occur. In this case, it can be solved as long as you avoid naming duplication. The most common way is to agree on a prefix.

The purpose of using namespace:

When working on a team project, avoid conflicts with classes created by other team members; when individuals are responsible for the project, Avoid conflicts between newly created classes;

According to personal understanding, when using the required class, it needs to be introduced by require or include first, so the premise for a class redefinition error to occur is: two identically named classes Classes have been introduced. Currently, some PHP frameworks will automatically load (i.e. include) all newly created model classes, so in order to avoid name conflicts between your new model classes and the native core classes of the project framework, namespace is used. (After thinking about it, conflicts with new classes created by team members should be avoided through communication. Even after the incident, the class name should be readjusted and maintained immediately to avoid increased maintenance complexity caused by confusion in the understanding of the class later.)

Combined the usage methods to further understand its purpose.

How to use namespace:

For testing, I will create 3 files: 1.php and name.php (this file Used to perform tests), which will not be explained later. Please pay attention to the code changes yourself.

1. The definition of the name after namespace is not case-sensitive

namespace one;
namespace One;
namespace ONE;

Any of the above writing methods are acceptable, just choose one as your own standard. (I will use the first method for testing in the following code)

2. If there is no namespace defined, it is understood that the top-level namespace is used. When creating a new class, you can add a backslash\ before the class or not.

//1.php 
class Person{
 function __construct(){
  echo 'I am one!';
 }
}
//name.php
require_once './1.php';
new Person(); //输出 I am one!;
new \Person(); //输出 I am one!;

3. When adding a namespace to a new class, backslash characters must be used instead of forward slashes. Bar.

Memory method: Find the slashes in % in the order they are interpreted as forward slashes. (Sometimes when I say backslash, I don’t know which direction it is. I used to remember it in the ascending direction from left to right, but now I feel that this is too unreliable)

//name.php
require_once './1.php';
new /Person(); // 代码报错：Parse error: syntax error, unexpected '/'

4. The class is in the specified namespace. When new class, be sure to bring the specified namespace.

Without the specified namespace, according to point 2, PHP will find this class from the top-level namespace. Remember: This cannot be understood as the top-level namespace includes all other namespaces. Instead, the top-level namespace should be completely separated from other namespaces.

//1.php 
namespace one;
class Person{
 function __construct(){
  echo 'I am one!';
 }
}
//name.php
require_once './1.php';
new \one\Person(); //输出 I am one!;
new \Person(); //代码报错：Fatal error: Class 'Person' not found

You can take this popular example to understand: bring the specified namespace to represent someone’s apple (in his hand), and the top-level namespace represents the apple. Apples in the box (in the box). Now if you want to find someone's apple, you need to bring someone's namespace. Otherwise, you will look for someone's apple from the box, and of course you can't find it.

5. The code after the namespace declaration belongs to this namespace, even if there is include or require, it will not be affected (the focus is on the understanding of the second half of the sentence, see the code for details).

//1.php 
namespace one;
class Person{
 function __construct(){
  echo 'I am one!';
 }
}
//name.php
namespace test;
require './1.php'; 
new \one\Person(); //输出 I am one!;
new Person(); //这里结果会是什么呢，猜猜看

The last line results in an error:

Fatal error: Class 'test\Person' not found

First of all, here is the Compare the second point:

The second point, I said, when there is no namespace, when there is a new class, the meaning is the same whether there is a backslash or not.

Here, with the namespace, the meaning of having and not having backslashes is different.

Replace the last line with

new \Person();

and the result is an error:

Fatal error: Class 'Person' not found

Then, let’s talk about the current point.

We can find that the namespace corresponding to the last line of code is test and is not affected by the namespace in the require file.

To further strengthen the verification, I modified the name.php file as follows:

//name.php
namespace test;
require './1.php'; 
class Person{
 function __construct(){
  echo 'I am test!';
 }
}
new \one\Person(); //输出 I am one!;
new Person(); //这里结果会是什么，自己猜猜看

Finally, this example refreshed my understanding of require. .

According to my previous understanding of require: before the PHP program is executed, it will first read in the file specified by require, making it a part of the PHP program web page. So I often simply understand it as replacement, which is just putting the extracted code back into its original place. Then I tried putting the contents of the 1.php file into name.php:

//name.php
namespace test;
namespace one;
class Person{
 function __construct(){
  echo 'I am one!';
 }
}
class Person{
 function __construct(){
  echo 'I am test!';
 }
}

Without the new class, the file will report an error:

Fatal error: Cannot redeclare class one\Person

It seems that simply understanding require as replacement does not work here.

6. The namespace does not contain the class name. Even if there is a part with the same name as the class name, it does not represent the class. new class, you still have to bring this part.

//name.php
namespace test\person;
class Person{
 function __construct(){
  echo 'I am test!';
 }
}new \test\person\Person();  //命名空间里person无法代表类名

But this is purely superfluous, just go ahead and don’t include the class name in the namespace.

7. Multiple namespaces can exist in a php file, and there cannot be any code before the first namespace.

只说第一个命名空间前不能有任何代码，之后的命名空间之前可以有代码。这个自行测试即可。

//name.php
namespace test;
echo 'zhai14';
namespace zhai;
require './1.php';

php命名空间namespace告一段落了，接下来说说use的用途。

使用use的目的：

在命名空间字符串过长时，使用use可以相应的缩短命名空间。

use的使用方法：

1.new类时，最前面无需用反斜杠。此外，use后没有as时，缩短的命名空间默认为最后一个反斜杠后的内容。

//name.php
namespace animal\dog;
class Life{
 function __construct(){
  echo 'dog life!';
 }
}
namespace animal\cat;
class Life{
 function __construct(){
  echo 'cat life!';
 }
}
new Life(); //按照代码执行顺序，这里默认animal\cat这个命名空间
new \animal\dog\Life(); //A
use animal\dog; //a
new dog\Life(); //B
use animal\dog as d; //b
new d\Life();

 通过A、B行代码比较，需要注意：

使用use后，new类时，最前面没有反斜杠。

没使用use时，命名空间最前面有反斜杠

通过a、b行代码比较，可以理解：

use后没有as时，缩短的命名空间默认为最后一个反斜杠后的内容。如上的:

use animal\dog;

相当于

use animal\dog as dog; 

2.namespace后面不建议加类名，但use后可以。

//name.php
namespace animal\dog;
class Life{
 function __construct(){
  echo 'dog life!';
 }
}
namespace animal\cat;
class Life{
 function __construct(){
  echo 'cat life!';
 }
}
use animal\dog\Life as dog; 
new dog();

如上所示，use后加上类名后，就相当于把类改了个名称：由Life改为dog了。

上面不用as dog就会报错：

Fatal error:  Cannot use animal\dog\Life as Life because the name is already in use
因为cat下也有个一样名称的Life类。

可以理解为，使用use后，这个昵称对应的类只能归当前命名空间占有，其它命名空间下不允许存在该类。

//name.php
namespace animal\dog;
class Life{
 function __construct(){
  echo 'dog life!';
 }
}
class Dog{
 function __construct(){
  echo 'dog in dog!';
 }
}
namespace animal\cat;
// class Dog{
// function __construct(){
//  echo 'dog in cat!';
//  }
// }
class Life{
 function __construct(){
  echo 'cat life!';
 }
}
use animal\dog; 
new dog\Dog();

如上，使用了

use animal\dog;
cat

通过上面代码，我想使用use的目的效果（缩短命名空间名称）就很明显了。

简单总结一下：

namespace就是划分领域的作用，代表这些东西是属于某个命名空间下的。

use就是起小名的作用，不论写起来还是说起来都可以省不少事儿。

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