Complete usage of PHP command space

We know that in namespaceone of the clearest purposes is to solve the problem of duplicate names, so in PHP, two functions or classes are not allowed to have the same name, otherwise 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 creating new classes before and after There is a conflict between classes;

According to my 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 classes with the same name have be introduced. Currently, some PHP frameworks will

automatically load (i.e. include) all newly created model classes, so in order to avoid duplicate 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 with the usage methods to further understand its purpose.

How to use namespace:

In order to test, I will create 3 files: 1.php and name.php (this file is used to perform testing), which will not be explained later. Please note the code changes yourself.

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

namespace one;

namespace One;
namespace ONE;

You can write it the same way as above, choose Just use 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.

Memory method: Find the order of slashes in % and interpret them as forward slashes. (Sometimes when talking about backslashes, I don’t even 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. Classes are specified in Under the namespace, when adding a new class, the specified namespace must be included.

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, top-level namespaces 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: bringing the specified namespace represents someone's apples (in his hand), and the top-level namespace represents the apples in the apple 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, compare this with the second point:

The second point, I said, when there is no namespace, when the new class , the meaning is the same with or without the backslash.

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 to put 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 will 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(); //person cannot represent the class name in the namespace

But this is purely superfluous, just don’t include the class name in the namespace Just fine.

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

It only says that there cannot be any code before the first namespace, and there can be code before subsequent namespaces. You can test this yourself.

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

php namespacenamespace has come to an end, let’s talk about the use of use.

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。

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