php - php手册中关键字 "Classes must be defined before they are used "如何理解 "extends"

Question

我得到 2 个 php 文件一个.php

<?php
class A extends B {}
class B{}

php a.php
>>>没有错误

b.php

<?php
class A extends B {}
class B extends C {}
class C{}

php b.php
>>>> fatal error :在第 2 行的 b.php 中找不到类“B”

根据 [php.net manual] :

Classes must be defined before they are used! If you want the class Named_Cart to extend the class Cart, you will have to define the class Cart first. If you want to create another class called Yellow_named_cart based on the class Named_Cart you have to define Named_Cart first. To make it short: the order in which the classes are defined is important.

所以任何人请解释为什么 a.php 没有“ fatal error ”。

Answer 1

根据 PHP Inheritence文档。

Unless autoloading is used, then classes must be defined before they are used. If a class extends another, then the parent class must be declared before the child class structure. This rule applies to classes that inherit other classes and interfaces.

这个例子没有类 B 和 D 的依赖:

<?php
 //This will work
 class A extends B {} //searches for class B in current document
 class B {}

 class C extends D{}
 class D{}

B类依赖于C类

<?php
 //This will not work
 class A extends B {} //searches for class B in current document but class B has dependencies
 class B extends C {}
 class C{}

对于包含和要求，您必须先包含该文件，然后才能在该文件中使用声明的函数/类。

另见 autoloading .