字段是用于描述类的么个方面的性质。
字段是用于描述类的某个方面的性质。它与一般的PHP 变量非常相似,只是有一些细微的差别,本节将介绍这些差别。这一节还将讨论如何声明和使用字段,下一节则介绍如何使用字段的作用域来进行访问限制。
声明字段
字段声明的有关规则与变量声明的规则非常类似;实际上,可以说没有区别。因为PHP 是松散类型的语言,字段甚至不需要声明;可以由类对象同时创建和赋值,但很少会这样做。相反,常见的做法是在类开始处声明字段。此时可以为字段赋初值。示例如下:
class Employee
{
public $name="John";
private $wage;
}
在这个例子中,两个字段name 和wage 前面都有作用域描述符(public 或Private ) ,这是声明字段时的常用做法。声明之后,每个字段都可以在作用域描述符所指示的范围内使用。如果你不了解作用域对于类字段有何作用,不要担心,后面将会介绍这个问题。
使用字段
与变量不同,字段要使用->操作符引用,而不是使用美元符。此外,因为字段的值一般是给定对象所特有的,所以它与那个对象具有如下的相互关系:
$object->field
例如,在本章开始时描述Employee类包括字段name、title和wage .如果创建了一个名为$employee的Employee类型对象,就可以如下引用这些字段:
$employee->name
$employee->title
$employee->wage
在定义字段的类中引用字段时,还要使用->操作符,但此时不使用相应的类名,而是使用$this 关键字。$this表示要引用当前类(要访问或操作的字段所在的类)中的字段。因此,如果要在上述Employee类中创建一个设置姓名字段的方法,则如下所示:
function setName($name)
{
$this->name=$name;
}

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