Section 1: Overview of namespaces
Version requirements
PHP introduces namespaces starting from version 5.3.0
What is a namespace
Quoting the example in the PHP official manual:
The file foo.txt can be in the directory / at the same time exists in home/greg and /home/other, but two foo.txt files cannot exist in the same directory. Additionally, 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.
The role of namespaces
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:
User-written code and PHP Name conflicts between internal classes/functions/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.
Example of namespace syntax
<?phpnamespace my\name; // 参考 "定义命名空间" 小节
class MyClass {}
function myfunction() {}
const MYCONST = 1;
$a = new MyClass;
$c = new \my\name\MyClass; // 参考 "全局空间" 小节
$a = strlen('hi'); // 参考 "使用命名空间：后备全局函数/常量" 小节
$d = namespace\MYCONST; // 参考 "namespace操作符和__NAMESPACE__常量” 小节
$d = __NAMESPACE__ . '\MYCONST';
echo constant($d); // 参考 "命名空间和动态语言特征" 小节?>Section 2 Definition of namespace
Definition of namespace
Although any legal PHP code can be included in a namespace, only the following types of code are affected by namespaces, they They are: classes (including abstract classes and traits), interfaces, 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 except one: the declare keyword.
The only legal code before declaring a namespace is the declare statement that defines how the source file is encoded. In addition, all non-PHP code, including whitespace, cannot appear before a namespace declaration.
In addition, unlike other language features of PHP, the same namespace can be defined in multiple files, which allows the contents of the same namespace to be divided and stored in different files.
Example of correct namespace:
<?phpnamespace MyProject;
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }
?>Example of incorrect 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 ?>
Defining 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.
The following example creates the constant MyProjectSubLevelCONNECT_OK, the class MyProjectSubLevelConnection and the function MyProjectSubLevelconnect.
<?php
namespace MyProject\Sub\Level;
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }
?>Define multiple namespaces in the same file
You can also define multiple namespaces in the same file. There are two syntaxes for defining multiple namespaces in the same file.
The recommended syntax is to use braces to define the namespace.
<?php
namespace MyProject {
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }
}
namespace AnotherProject {
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */ }
}?>In actual programming practice, it is highly discouraged to define multiple namespaces in the same file. This method is mainly used to combine multiple PHP scripts in the same file.
Except for the opening declare statement, there must be no PHP code outside the namespace brackets.
The above is the content of Note 007 PHP namespace - the first part. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!
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