Here is some clarification about PHP inheritance – there is a lot of bad information on the net. PHP does support Multi-level inheritance. (I tested it using version 5.2.9). It does not support multiple inheritance.
This means that you cannot have one class extend 2 other classes (see the extends keyword). However, you can have one class extend another, which extends another, and so on.
Example:
<?php
class A {
// more code here
}
class B extends A {
// more code here
}
class C extends B {
// more code here
}
$someObj = new A(); // no problems
$someOtherObj = new B(); // no problems
$lastObj = new C(); // still no problems
?>
객체 상속
상속은 정착된 프로그래밍 원칙이며, PHP는 이 원칙을 객체모델에서 사용하고 있습니다. 이 원칙은 많은 클래스와 객체들을 연관짓는 방법에 영향을 미칩니다.
예를 들면, class 를 확장했을때, subcalss 는 부모의 클래스로부터 모든 public, protected 메서드를 상속받게 됩니다. 자식 클래스가 메서드를 재정의 하지 않는한, 원래 기능이 유지 됩니다.
상속은 추상화된 기능을 정의하는데 유용합니다. 그리고, 공유된 기능을 위해 비슷한 객체들마다 기능을 재정의 하는 수고를 덜어줍니다.
Note:
자동로딩이 사용되지 않는한, 클래스는 사용되기 이전에 정의되어 있어야 합니다. 클래스가 다른것으로 확장된다면, 자식 클래스 구조보다 부모클래스가 앞서 선언되어 있어야 합니다. 이 규칙은 다른 클래스과 인터페이스를 상속받는 클래스들에게도 적용됩니다.
Example #1 상속 예제
<?php
class Foo
{
public function printItem($string)
{
echo 'Foo: ' . $string . PHP_EOL;
}
public function printPHP()
{
echo 'PHP is great.' . PHP_EOL;
}
}
class Bar extends Foo
{
public function printItem($string)
{
echo 'Bar: ' . $string . PHP_EOL;
}
}
$foo = new Foo();
$bar = new Bar();
$foo->printItem('baz'); // Output: 'Foo: baz'
$foo->printPHP(); // Output: 'PHP is great'
$bar->printItem('baz'); // Output: 'Bar: baz'
$bar->printPHP(); // Output: 'PHP is great'
?>
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User Contributed Notes 8 notes
jackdracona at msn dot com ¶
14 years ago
Mohammad Istanbouly ¶
7 years ago
I think the best way for beginners to understand inheritance is through a real example so here is a simple example I can gave to you
<?php
class Person
{
public $name;
protected $age;
private $phone;
public function talk(){
//Do stuff here
}
protected function walk(){
//Do stuff here
}
private function swim(){
//Do stuff here
}
}
class Tom extends Person
{
/*Since Tom class extends Person class this means
that class Tom is a child class and class person is
the parent class and child class will inherit all public
and protected members(properties and methods) from
the parent class*/
/*So class Tom will have these properties and methods*/
//public $name;
//protected $age;
//public function talk(){}
//protected function walk(){}
//but it will not inherit the private members
//this is all what Object inheritance means
}
strata_ranger at hotmail dot com ¶
14 years ago
I was recently extending a PEAR class when I encountered a situation where I wanted to call a constructor two levels up the class hierarchy, ignoring the immediate parent. In such a case, you need to explicitly reference the class name using the :: operator.
Fortunately, just like using the 'parent' keyword PHP correctly recognizes that you are calling the function from a protected context inside the object's class hierarchy.
E.g:
<?php
class foo
{
public function something()
{
echo __CLASS__; // foo
var_dump($this);
}
}
class foo_bar extends foo
{
public function something()
{
echo __CLASS__; // foo_bar
var_dump($this);
}
}
class foo_bar_baz extends foo_bar
{
public function something()
{
echo __CLASS__; // foo_bar_baz
var_dump($this);
}
public function call()
{
echo self::something(); // self
echo parent::something(); // parent
echo foo::something(); // grandparent
}
}
error_reporting(-1);
$obj = new foo_bar_baz();
$obj->call();
// Output similar to:
// foo_bar_baz
// object(foo_bar_baz)[1]
// foo_bar
// object(foo_bar_baz)[1]
// foo
// object(foo_bar_baz)[1]
?>
akashwebdev at gmail dot com ¶
9 years ago
The Idea that multiple inheritence is not supported is correct but with tratits this can be reviewed.
for e.g.
<?php
trait custom
{
public function hello()
{
echo "hello";
}
}
trait custom2
{
public function hello()
{
echo "hello2";
}
}
class inheritsCustom
{
use custom, custom2
{
custom2::hello insteadof custom;
}
}
$obj = new inheritsCustom();
$obj->hello();
?>
jarrod at squarecrow dot com ¶
15 years ago
You can force a class to be strictly an inheritable class by using the "abstract" keyword. When you define a class with abstract, any attempt to instantiate a separate instance of it will result in a fatal error. This is useful for situations like a base class where it would be inherited by multiple child classes yet you want to restrict the ability to instantiate it by itself.
Example........
<?php
abstract class Cheese
{
//can ONLY be inherited by another class
}
class Cheddar extends Cheese
{
}
$dinner = new Cheese; //fatal error
$lunch = new Cheddar; //works!
?>
niemans at pbsolo dot nl ¶
4 years ago
Inheritance works at create time, i.e. using the keyword 'new'. Static properties confused my understanding, so in order tho show the effect of visibility to inherintence I've created a simple demo script along with some set and get magic:
<?php
class A {
private $a = 'private';
protected $b = 'protected';
public $c = 'public';
static $d = 'static';
public function __construct()
{
$this->e = 'constructed';
}
public function __set($property, $value)
{
echo ' set ' . $property . '=' . $value;
$this->$property=$value;
}
public function __get($property)
{
echo ' get ' . $property;
$this->$property = 'dynamic'; // invokes __set() !!
return $this->$property;
}
}
class B extends A
{
public function constructMe()
{
$this->e = 'constructed2';
}
}
class C extends B
{
public function __construct()
{
parent::constructMe();
}
}
echo " \n";
$a = new A();
$b = new B();
echo " \n";
echo ' B:c='.$b->c;
echo " \n";
echo ' B:d=' .$b->d;
echo " \n";
$c = new C();
echo " \n";
print_r($a);
print_r($b);
print_r($c);
print_r(A::$d);
print_r(B::$d);
print_r(C::$d);
echo 'A class: ';
$R = new reflectionclass('A');
print_r($R->getdefaultproperties());
print_r($R->getstaticproperties());
echo 'B class: ';
$R = new reflectionclass('B');
print_r($R->getdefaultproperties());
print_r($R->getstaticproperties());
?>
This outputs:
set e=constructed
B:c=public
get d set d=dynamic B:d=dynamic
set e=constructed2
A Object
(
[a:A:private] => private
[b:protected] => protected
[c] => public
[e] => constructed
)
B Object
(
[a:A:private] => private
[b:protected] => protected
[c] => public
[d] => dynamic
)
C Object
(
[a:A:private] => private
[b:protected] => protected
[c] => public
[e] => constructed2
)
staticstaticstaticA class: Array
(
[d] => static
[a] => private
[b] => protected
[c] => public
)
Array
(
[d] => static
)
B class: Array
(
[d] => static
[b] => protected
[c] => public
)
Array
(
[d] => static
)
This shows how private variables ($a) are inherited, how static variables ($d) are inherited (by the class, not by the object) and that changing or adding variables in the parent ($e, $d) are not inherited by the child.
Anonymous ¶
5 years ago
PHP7 gives you a warning if you redeclare a function in a child class with different parameters. For example:
class foo {
function print($text='') {
print text;
}
}
class bar extends foo {
function print($text1='',$text2='') {
print text1.text2
}
}
will give a PHP Warning: Declaration of bar::print($text1 = '', $text2 = '') should be compatible with foo::print($text= '').
sibian0218 at gmail dot com ¶
6 years ago
I've noticed one thing concerning inheritance...
When declaring an abstract class with a private method,
which is overridden by a sub-class, private takes precedence over public for child class...
(in the case you're redeclaring a method with a different signature in fact).
Hope this helps
