提出
在命名空间提出之前，不同的组件很容易碰到命名的冲突，例如 Request 、Response 等常见的命名。PHP 在 5.3 后提出了命名空间用来解决组件之间的命名冲突问题，主要参考了文件系统的设计：
同一个目录下不允许有相同的文件名 - 同一个命名空间下不允许有相同的类；
不同的目录可以有同名文件 - 不同的命名空间可以有相同的类；
定义
使用 namespace 关键字来定义一个命名空间。其中，顶层命名空间通常为厂商名，不同开发者的厂商命名空间是唯一的。命名空间不需要与文件目录一一对应，但是最好遵守 PSR-4 规范。
<?php
namespace Symfony\Component\HttpFoundation;
class Request {
}命名空间必须在所有代码之前声明，唯一的例外就是 declare 关键字。
<?php declare(strict_types=1); namespace App;
命名空间内可包含任意 PHP 代码，但是仅对类 (包括抽象类和 Trait)、接口、函数和常量这四种类型生效。
<?php
namespace MyProject;
const CONNECT_OK = 1;
class FOO {}
interface Foo{}
function foo() {}使用
使用 use 关键字来引入命名空间
<?php
namespace App;
use Symfony\Component\HttpFoundation\Request;
use Foo\Bar;
class Test {
public function run()
{
$bar = new Bar();
}
}定义和使用推荐遵循 PSR-2 的规范
namespace 之后必须存在一个空行；
所有 use 声明必须位于 namespace 声明之后；
每条 use 声明必须只有一个 use 关键字。
use 语句块之后必须存在一个空行。
当 use 引入的类出现同名时，可使用 as 来定义别名
<?php
namespace App;
use Foo\Bar as BaseBar;
class Bar extends BaseBar {
}限定符
除了使用 use 外，还可以直接使用 \ 限定符来进行解析，规则很简单：如果含有 \ 前缀则代表从全局命名空间开始解析，否则则代表从当前命名空间开始解析。
<?php namespace App; \Foo\Bar\foo(); // 解析成 \Foo\Bar\foo(); Foo\Bar\foo(); // 解析成 App\Foo\Bar\foo();
此规则也适用于函数、常量等
$a = \strlen('hi'); // 调用全局函数 strlen $b = \INI_ALL; // 访问全局常量 INI_ALL $c = new \Exception('error'); // 实例化全局类 Exception
有两个需要特别注意的地方：
对于函数和常量而言，如果当前命名空间不存在，则会自动去全局命名空间去寻找，因此可省略 \ 前缀。对于类而言，如果当前命名空间解析不到，不会去全局空间寻找，因此，不可省略 \
$a = strlen('hi'); $b = INI_ALL; $c = new Exception('error'); // 错误 $c = new \Exception('error'); // 正确
当动态调用命名空间时，该命名空间始终会被当成是全局命名空间，因此可以省略前缀 \
$class1 = 'Foo\Bar'; $object1 = new $class1; // 始终被解析成 \Foo\Bar
在内部访问命名空间
PHP 支持两种抽象的访问当前命名空间内部元素的方法，__NAMESPACE__ 魔术常量和 namespace 关键字。
__NAMESPACE__ 常量的值是包含当前命名空间名称的字符串，如果是在全局命名空间，则返回空字符串。
<?php
namespace MyProject;
function get($classname)
{
$a = __NAMESPACE__ . '\\' . $classname;
return new $a;
}关键字 namespace 可用来显式访问当前命名空间或子命名空间中的元素。它等价于类中的 self 操作符
namespace App; use blah\blah as mine; blah\mine(); // App\blah\mine() namespace\blah\mine(); // App\blah\mine() namespace\func(); // App\func() namespace\sub\func(); // App\sub\func() namespace\cname::method(); // App\cname::method() $a = new namespace\sub\cname(); // App\sub\cname $b = namespace\CONSTANT; // App\CONSTANT
转义 \ 符号
此外，推荐对所有的 \ 进行转义，避免出现不可预期的后果
$class = "dangerous\name"; // \n 被解析成换行符 $obj = new $class; $class = 'dangerous\name'; // 正确，但是不推荐 $class = 'dangerous\\name'; // 推荐 $class = "dangerous\\name"; // 推荐
The above is the detailed content of PHP core features namespace. For more information, please follow other related articles on the PHP Chinese website!
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