PHP namespace overview and definition
Namespace Overview
What is a namespace? 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. For example, the file foo.txt can exist in the directories /home/greg and /home/other at the same time, but two foo.txt files cannot exist in the same directory. In addition, when accessing the foo.txt file outside the directory /home/greg, we must put the directory name and directory separator before the file name to get /home/greg/foo.txt. The application of this principle to the field of programming is the concept of namespace.
In PHP, namespaces are used to solve two types of problems encountered when creating reusable code such as classes or functions when writing class libraries or applications:
Code written by users and classes/functions inside PHP Name conflicts between constants or third-party classes/functions/constants.
Create an alias (or short) name for a very long identifier name (usually defined to alleviate the first type of problem), improving the readability of the source code.
PHP namespaces provide a way to group related classes, functions and constants together. The following is an example illustrating PHP namespace syntax:
Example #1 Namespace syntax example
<?php
namespace my\name;
class MyClass {}
function myfunction() {}
const MYCONST = 1;
$a = new MyClass;
$c = new \my\name\MyClass;
$a = strlen('hi');
$d = namespace\MYCONST;
$d = __NAMESPACE__ . '\MYCONST';
echo constant($d);
?>PHP supports namespaces in versions after 5.3.0.
Define namespaces
While any legal PHP code can be included in a namespace, there are only three types of code affected by namespaces, and they are: classes, functions, and constants.
The namespace is declared through the keyword namespace. If a file contains a namespace, it must declare the namespace before all other code.
Example #1 Declare a single namespace
<?php
namespace MyProject;
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }
?>The only legal code before declaring a namespace is the declare statement that defines the encoding of the source file. In addition, all non-PHP code including whitespace characters cannot appear before the namespace declaration:
Example #2 Declare a single namespace
<html>
<?php
namespace MyProject; // 致命错误 - 命名空间必须是程序脚本的第一条语句
?>In addition, unlike other PHP language features, the same namespace can be defined in multiple file, which allows the contents of the same namespace to be split and stored in different files.
Define sub-namespaces
Much like the relationship between directories and files, PHP namespaces also allow you to specify hierarchical namespace names. Therefore, namespace names can be defined in a hierarchical manner:
Example #1 Declare a hierarchical single namespace
<?php
namespace MyProject\Sub\Level;
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }
?>The above example creates the constant MyProjectSubLevelCONNECT_OK, the class MyProjectSubLevelConnection and the function MyProjectSubLevelConnection.
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