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집 > php教程 > php手册 > PHP 프론트엔드 학습을 위한 객체지향 시리즈 4부의 키워드

PHP 프론트엔드 학습을 위한 객체지향 시리즈 4부의 키워드

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풀어 주다： 2016-11-16 10:24:02
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[1]public [2]protected [3]private[4]final[5]static[6]const[7]this[8]self[9]parent

前面的话

　　php实现面向对象的一个显著特征是大量使用关键字，本文将详细介绍关键字

 

public

　　public表示公有，它具有最大的访问权限，被定义为公有的类成员可以在任何地方被访问

　　如果属性用 var 定义，则被视为公有，如果方法没有设置关键字，则该方法默认为公有

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">public</span> <span style="color: #800080;">$public</span> = 1<span style="color: #000000;">;
    </span><span style="color: #0000ff;">function</span> test(<span style="color: #800080;">$var</span><span style="color: #000000;">){
        </span><span style="color: #0000ff;">echo</span> "{<span style="color: #800080;">$var</span>}000"<span style="color: #000000;">;
    }
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo;
</span><span style="color: #800080;">$d1</span>->test(<span style="color: #800080;">$d1</span>-><span style="color: #0000ff;">public</span>);<span style="color: #008000;">//</span><span style="color: #008000;">1000</span>
?>
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protected

　　protected表示受保护的，被定义为受保护的类成员则可以被其自身以及其子类和父类访问

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">protected</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> fn(){
        </span><span style="color: #0000ff;">echo</span> '111'<span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">class</span> demo1 <span style="color: #0000ff;">extends</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
        parent</span>::<span style="color: #000000;">fn();
    }    
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo1;
</span><span style="color: #800080;">$d1</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">111</span>
?>
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private

　　private表示私有的，被定义为私有的类成员则只能被其定义所在的类访问

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> demo{
    </span><span style="color: #0000ff;">private</span> <span style="color: #800080;">$private</span> = 1<span style="color: #000000;">;
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
        </span><span style="color: #0000ff;">echo</span>(<span style="color: #800080;">$this</span>-><span style="color: #0000ff;">private</span><span style="color: #000000;">);
    }
}
</span><span style="color: #800080;">$d1</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> demo;
</span><span style="color: #800080;">$d1</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">1</span>
?>
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final

　　PHP5新增了final关键字，它只能用来修饰类和方法，不能使用final这个关键字来修饰成员属性，因为final是常量的意思，我们在PHP里定义常量使用的是define()函数和const关键字，所以不能使用final来定义成员属性

　　如果父类中的方法被声明为final，则子类无法覆盖该方法。如果一个类被声明为 final，则不能被继承

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> BaseClass {
   </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> test() {
       </span><span style="color: #0000ff;">echo</span> "BaseClass::test() called\n"<span style="color: #000000;">;
   }
   </span><span style="color: #0000ff;">final</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> moreTesting() {
       </span><span style="color: #0000ff;">echo</span> "BaseClass::moreTesting() called\n"<span style="color: #000000;">;
   }
}
</span><span style="color: #0000ff;">class</span> ChildClass <span style="color: #0000ff;">extends</span><span style="color: #000000;"> BaseClass {
   </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> moreTesting() {
       </span><span style="color: #0000ff;">echo</span> "ChildClass::moreTesting() called\n"<span style="color: #000000;">;
   }
}
</span><span style="color: #008000;">//</span><span style="color: #008000;"> Results in Fatal error: Cannot override final method BaseClass::moreTesting()</span>
?>
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static

　　static关键字表示静态的意思，用于修饰类的成员属性和成员方法（即为静态属性和静态方法）

　　类中的静态属性和静态方法不用实例化(new)就可以直接使用类名访问

　　[注意]静态属性不能通过一个类已实例化的对象来访问，但静态方法可以

　　由于静态方法不需要通过对象即可调用，所以伪变量 $this 在静态方法中不可用，静态属性不可以由对象通过 -> 操作符来访问

　　用静态方式调用一个非静态方法会导致一个 E_STRICT 级别的错误

　　就像其它所有的 PHP 静态变量一样，静态属性只能被初始化为文字或常量，不能使用表达式。所以可以把静态属性初始化为整数或数组，但不能初始化为另一个变量或函数返回值，也不能指向一个对象

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> Foo
{
    </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #800080;">$my_static</span> = 'foo'<span style="color: #000000;">;
    </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> staticValue() {
        </span><span style="color: #0000ff;">return</span> self::<span style="color: #800080;">$my_static</span><span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">class</span> Bar <span style="color: #0000ff;">extends</span><span style="color: #000000;"> Foo
{
    </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> fooStatic() {
        </span><span style="color: #0000ff;">return</span> parent::<span style="color: #800080;">$my_static</span><span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">print</span> Foo::<span style="color: #800080;">$my_static</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #800080;">$foo</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Foo();
</span><span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>->staticValue() . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>::<span style="color: #800080;">$my_static</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'foo'</span>
<span style="color: #0000ff;">print</span> <span style="color: #800080;">$foo</span>->my_static . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;">报错 </span>
?>
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const

　　可以把在类中始终保持不变的值定义为常量。在定义和使用常量的时候不需要使用$符号，而是使用const

　　常量的值必须是一个定值，不能是变量，类属性，数学运算的结果或函数调用

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass
{
    </span><span style="color: #0000ff;">const</span> <span style="color: #008080;">constant</span> = 'constant value'<span style="color: #000000;">;
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> showConstant() {
        </span><span style="color: #0000ff;">echo</span>  self::<span style="color: #008080;">constant</span> . "\n"<span style="color: #000000;">;
    }
}
</span><span style="color: #0000ff;">echo</span> MyClass::<span style="color: #008080;">constant</span> . "\n";<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #800080;">$classname</span> = "MyClass"<span style="color: #000000;">;
</span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$classname</span>::<span style="color: #008080;">constant</span> . "\n"; <span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #800080;">$class</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> MyClass();
</span><span style="color: #800080;">$class</span>->showConstant();<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
<span style="color: #0000ff;">echo</span> <span style="color: #800080;">$class</span>::<span style="color: #008080;">constant</span>."\n";<span style="color: #008000;">//</span><span style="color: #008000;">'constant value'</span>
?>
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this

　　当一个方法在类定义内部被调用时，有一个可用的伪变量this，特殊对象的引用this就是在对象内部的成员方法中，代表本对象的一个引用，但只能在对象的成员方法中使用，不管是在对象内部使用$this访问自己对象内部成员。还是在对象外部通过对象的引用名称访问对象中的成员，都需要使用特殊的运算符“->”来完成访问

　　[注意]this在静态方法中不可用

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> A
{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> foo()
    {
        </span><span style="color: #0000ff;">if</span> (<span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$this</span><span style="color: #000000;">)) {
            </span><span style="color: #0000ff;">echo</span> '$this is defined ('<span style="color: #000000;">;
            </span><span style="color: #0000ff;">echo</span> <span style="color: #008080;">get_class</span>(<span style="color: #800080;">$this</span><span style="color: #000000;">);
            </span><span style="color: #0000ff;">echo</span> ")\n"<span style="color: #000000;">;
        } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> {
            </span><span style="color: #0000ff;">echo</span> "\$this is not defined.\n"<span style="color: #000000;">;
        }
    }
}
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> B
{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> bar()
    {
        </span><span style="color: #008000;">//</span><span style="color: #008000;"> Note: the next line will issue a warning if E_STRICT is enabled.</span>
        A::<span style="color: #000000;">foo();
    }
}

</span><span style="color: #800080;">$a</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> A();
</span><span style="color: #800080;">$a</span>->foo();<span style="color: #008000;">//</span><span style="color: #008000;">$this is defined (A) </span>
A::foo();<span style="color: #008000;">//</span><span style="color: #008000;">$this is not defined. </span>
<span style="color: #800080;">$b</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> B();
</span><span style="color: #800080;">$b</span>->bar();<span style="color: #008000;">//</span><span style="color: #008000;">$this is defined (B) </span>
B::bar();<span style="color: #008000;">//</span><span style="color: #008000;">$this is not defined.</span>
?>
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self

　　在类的方法中，不能用this来引用静态变量或静态方法，而需要用self来引用

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass
{
    </span><span style="color: #0000ff;">const</span> <span style="color: #008080;">constant</span> = 'constant value'<span style="color: #000000;">;
    </span><span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> showConstant() {
        </span><span style="color: #0000ff;">echo</span>  self::<span style="color: #008080;">constant</span> . "\n"<span style="color: #000000;">;
    }
}
</span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> MyClass;
</span><span style="color: #0000ff;">echo</span> <span style="color: #800080;">$var</span>->showConstant();<span style="color: #008000;">//</span><span style="color: #008000;">constant value</span>
?>
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parent

　　parent用于调用父类中定义的成员方法或常量

<?<span style="color: #000000;">php
</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> MyClass{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> fn(){
        </span><span style="color: #0000ff;">echo</span>('111'<span style="color: #000000;">);
    }
    </span><span style="color: #0000ff;">const</span> A = 'a'<span style="color: #000000;">;
}
</span><span style="color: #0000ff;">class</span> Class1 <span style="color: #0000ff;">extends</span><span style="color: #000000;"> MyClass{
    </span><span style="color: #0000ff;">function</span><span style="color: #000000;"> test(){
        </span><span style="color: #0000ff;">echo</span> parent::fn().parent::<span style="color: #000000;">A;
    }
}
</span><span style="color: #800080;">$var</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Class1;
</span><span style="color: #800080;">$var</span>->test();<span style="color: #008000;">//</span><span style="color: #008000;">111a</span>
?>
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