1、静态属性与方法
每一个类的实例拥有自己的属性和方法，每一个类也可以包含静态属性，静态属性不属于类的任何实例，可以把静态属性理解成存储在类中的全局变量，可以在任何地方通过类名引用静态属性。
1 <?php 2 class MyClass { 3 ????static $a = 0; ????4 ????function getValue(){ 5 ????????return self::$a; 6 ????} 7 } 8 ?9 $obj = new MyClass();10 echo $obj->getValue() . PHP_EOL;11 12 echo MyClass::$a . PHP_EOL;13 ?>2，静态属性的应用-----把一个唯一的id传递到类的所有实例中
<?php ???class MyClass { ???????static $count = 0; ???????public $unique_id; ???????function __construct(){ ???????????self::$count++; ???????????$this->unique_id = self::$count; ???????} ???} ???????$obj1 = new MyClass(); ???echo $obj1->unique_id . PHP_EOL; ???$obj2 = new MyClass(); ???echo $obj2->unique_id . PHP_EOL;?>3，静态方法
1 <?php 2 ????class HelloWorld { 3 ????????static function sayHello(){ 4 ????????????print "Hello World"; 5 ????????????self::printNewLine(); 6 ????????} 7 ?8 ????????static function printNewLine(){ 9 ????????????echo PHP_EOL;10 ????????}11 ????} ???12 ????HelloWorld::sayHello();13 ????HelloWorld::sayHello();14 ?>4，类的常量
>全局常量用 define函数定义
>类的常量与静态成员类似，他们属于类本身，而不是类的实例
>类的常量对大小写敏感
<?php ???class MyColorEnumClass { ???????const RED = "red"; ???????const GREEN = "green"; ???????const BLUE = "blue"; ???????static function printColor(){ ???????????print self::RED . PHP_EOL; ???????????print self::GREEN . PHP_EOL; ???????????print self::BLUE . PHP_EOL; ???????} ???} ???????????????MyColorEnumClass::printColor();?>5，对象克隆
在php5中，对象赋值，其实是引用 ， 如果需要拷贝对象，用克隆关键字clone
<?php ???class MyClass { ???????public $var = 1; ???} ???$obj1 = new MyClass(); ???//$obj2 = $obj1; ???$obj2 = clone $obj1; ???$obj2->var = 2; ???//使用$obj2 = $obj1, 下面输出2 ???//使用$obj2 = clone $obj1, 下面输出1 ???echo $obj1->var . PHP_EOL;?>6，多态
下面这个实例，如果想要再增加一种动物，那么需要改动2处，需要增加动物类和方法，而且需要在printTheRightSound方法中增加一个elseif分支，这种设计对于代码维护和分离非常不友好
<?php ???class Cat{ ???????function miao(){ ???????????echo "cat"; ???????} ???} ???class Dog{ ???????function wang(){ ???????????echo "dog"; ???????} ???} ???function printTheRightSound( $obj ){ ???????if( $obj instanceof Cat ) { ???????????$obj->miao(); ???????}else if ( $obj instanceof Dog ) { ???????????$obj->wang(); ???????}else { ???????????echo "传递了错误的对象类型"; ???????} ???????echo PHP_EOL; ???} ???printTheRightSound( new Cat() ); ???printTheRightSound( new Dog() );?>我们可以用继承的优势，改造上面的缺点
1 <?php 2 ????abstract class Animal { 3 ????????abstract function makeSound(); 4 ????} 5 ????class Cat extends Animal{ 6 ????????function makeSound(){ 7 ????????????echo "cat"; 8 ????????} 9 ????}10 ????class Dog extends Animal{11 ????????function makeSound(){12 ????????????echo "dog";13 ????????}14 ????}15 ????class Chicken extends Animal {16 ????????function makeSound(){17 ????????????echo "chicken";18 ????????}19 ????}20 21 ????function printTheRightSound( $obj ){22 ????????if( $obj instanceof Animal ) {23 ????????????$obj->makeSound();24 ????????}else {25 ????????????echo "传递了错误的对象类型";26 ????????}27 ????????echo PHP_EOL;28 ????}29 30 ????printTheRightSound( new Cat() );31 ????printTheRightSound( new Dog() );32 ????printTheRightSound( new Chicken() );33 ?>这样改造之后，printTheRightSound就不需要改动，只需要添加对应的动物类和方法即可
php面向对象精要(1)
原文地址：https://www.cnblogs.com/ghostwu/p/8458019.html