php namespacee principle

1. What is a namespace and how to define a namespace
Generally, there are no differences in a PHP program source code file or the same request. It is allowed to have more than two class names, constant names or function names with the same name. In this case, the program will report an error when running. However, in many practical situations, we cannot avoid having more than two class names, constant names, or function names with the same name in the same file. In this case, a namespace needs to be introduced.

Example:

<?php
function getUser () 
{
  return $username1;
}
  function getUser () 
  {
  return $username2;
  }

A program like this will report an error when executed. So we introduce the namespace, as follows:

<?php
namespace get1function getUser () 
{
  return $username1;
}
namespace get2
function getUser () 
{
  return $username2;
}

How to understand the meaning of the namespace? Taking the above code as an example, we imagine this PHP file as a folder, and the two functions inside as two files, but two files with the same name are not allowed in the same folder. , so we must divide them into two folders with different names. This is how namespaces work.

Special note:
Namespace is only valid for class names, function names, and constants (defined by const).
The front namespace of a file cannot have any valid characters in front of it

2. How to use namespace?
For example, how do we use getUser() under get2 defined above?
1) When accessing a function (or constant, class name), if the namespace is not specified, the proximity principle is used (access under the current namespace).
2) So, how to access the content under the specified namespace? As follows:

\get1\getUser(); // 访问get1下的getUser()方法

3. Multi-level namespace and three access methods
1) Multi-level namespace:
Imagine if a PHP project file is a file Folders, and there will be first-level subfolders under a folder, and there are subfolders (secondary subfolders) under the first-level subfolders. The same is true for namespaces. Under a namespace, there may be There is a namespace, which is the so-called multi-level namespace. For example, in thinkPHP:

namespace app\Index\Controller;

This is a multi-level namespace. The access method is the same as in the general name space.

2) Three access methods of namespace (similar to relative paths and absolute paths)
A. Unqualified name access method
B. Qualified name access method
C. Fully qualified Name access method

<?php
namespace app\get1
function getUser () 
{
  return $username1;
  }
  namespace get2
  function getUser () {
  return $username2;
  }
  getUser(); // 非限定名称访问方式
  \app\get1\getUser(); //完全限定名称访问方式，从根路径开始,类似绝对路径
  app\get1\getUser(); //限定名称访问方式，不是从根路径开始，类似相对路径

4. Namespace introduction mechanism
1) Space introduction: keyword use, note: when moving into the space, you must use the qualified name access method When accessing functions (or classes, constants) in the imported space, you cannot use unqualified names to access functions (or classes, constants) in the current namespace.
2) Introduction of space elements: keyword use. NOTE: Only classes can be introduced and then accessed using unqualified names.

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