One thing I figured out after a long time about extending a parent class that, if the child class does not have any construct function, it will use its parent's construct.
for example:
<?php
class Main
{
public $a;
public function __construct()
{
echo '::Parent Class initiated::';
$this -> a = 'we are in the parent class';
}
}
class Child extends Main
{
public function getA()
{
return $this -> a;
}
}
$main = new Main();
$main -> child = new Child;
echo $main -> child -> getA();
//Output - ::Parent Class initiated::::Parent Class initiated::we are in the parent class
?>
However, If we have a constructor in the child class as well:
<?php
class Child extends Main
{
public function __construct()
{
}
public function getA()
{
return $this -> a;
}
}
?>
Then :
<?php
$main = new Main();
$main -> child = new Child;
echo $main -> child -> getA();
// Output - ::Parent Class initiated::
?>
Note that the parent variable 'a' is not inherited by the child class if the constructor from the parent class isnt called.
This behaviour of extension made me waste a lot of my precious time, as I could not understand why some of my child classes were inheriting parent variables and some were not.
Hope this helps someone..
extends
他の既存のクラスに似た変数や関数を有するクラスが必要になることがよ くあります。実際、全てのプロジェクトで使用可能な一般的なクラスを定 義し、このクラスを特定のプロジェクトの各々の要求に合わせて調整する というのは、良いやり方です。これを簡単に行うにために、他のクラスを 拡張してクラス作成することが可能です。拡張あるいは派生クラスは、基 底クラスの全ての変数と関数を有します。(これは、実際には誰も亡くなっ ていませんが、'継承'と呼ばれます) この派生クラスには、派生クラスの 定義で追加したものも含まれます。クラスから定義を取り除く、つまり、 既存の関数や変数を未定義とすることはできません。派生クラスは、常に 単一の基底クラスに依存します。つまり、多重継承は、サポートされてい ません。クラスは、キーワード 'extends' を用いて拡張されます。
<?php
class Named_Cart extends Cart
{
var $owner;
function set_owner ($name)
{
$this->owner = $name;
}
}
?>
この例は、Cart の全ての変数及び関数に加えて変数$ownerと 関数 set_owner() を保持するクラス Named_Cart を定義しています。 この定義により、名前付きのカゴを通常の手段で作成し、カゴの保有者を 設定したり得たりすることができます。 名前付きのカゴで元のカゴクラスの関数を使うことも可能です。
$ncart = new Named_Cart; // 名前付きの籠を作成
$ncart->set_owner("kris"); // 籠の所有者の名前を設定
print $ncart->owner; // 籠の所有者を出力
$ncart->add_item("10", 1); // (籠から継承された機能)
"親と子"と呼ばれる関係もあります。ある親クラスを作成し、 この親クラスに基づく新しいクラス、つまり、子クラスを extendsにより作成します。 この新しい子クラスを使用することやこの子クラスに基づき他の クラスを作成することが可能です。
注意:
クラスは、使用される前に定義されている必要があります! Cartを拡張した クラス Named_Cartを作成したい場合、 まず、Cartを定義する必要があります。 クラスNamed_Cartに基づき Yellow_named_cartという名前の他のクラスを 作成する場合、まずNamed_Cartを定義する 必要があります。要するに、クラスの定義の順序は、重要です。
add a note
User Contributed Notes 22 notes
bilalghouri at live dot com ¶
14 years ago
bpotier at edreamers dot org ¶
23 years ago
Just one thing that may seem obvious but not mentionned in this page is that you need to include/require the file containing the parent class or else you'll get an error:
<?php
require(dirname(__FILE__).'/'.'myParent.php');
// ...
myChild extends myParent {
// ...
}
// ...
?>
Edward_nl ¶
18 years ago
If you are using a child-class. Remember to call the constructor of the parent class aswell before you start using it. Otherwise you might get different results then you expected. It is stated in this document, but I got confused by the given example. So, here is my example:
<?php
error_reporting(E_ALL);
class test {
var $var;
function test() {
$this->var = 3;
}
}
class testing extends test {
function testing() {
parent::test();
}
function My_test() {
return $this->var;
}
}
$p = new testing();
echo $p->My_test();
// Returns 3
f4bi0_ at hotmail dot com ¶
14 years ago
<?php
// what if we want to extend more then one class?
Abstract class ExtensionBridge
{
// array containing all the extended classes
private $_exts = array();
public $_this;
function __construct(){$_this = $this;}
public function addExt($object)
{
$this->_exts[]=$object;
}
public function __get($varname)
{
foreach($this->_exts as $ext)
{
if(property_exists($ext,$varname))
return $ext->$varname;
}
}
public function __call($method,$args)
{
foreach($this->_exts as $ext)
{
if(method_exists($ext,$method))
return call_user_method_array($method,$ext,$args);
}
throw new Exception("This Method {$method} doesn't exists");
}
}
class Ext1{
private $name="";
private $id="";
public function setID($id){$this->id = $id;}
public function setName($name){$this->name = $name;}
public function getID(){return $this->id;}
public function getName(){return $this->name;}
}
class Ext2{
private $address="";
private $country="";
public function setAddress($address){$this->address = $address;}
public function setCountry($country){$this->country = $country;}
public function getAddress(){return $this->address;}
public function getCountry(){return $this->country;}
}
class Extender extends ExtensionBridge
{
function __construct()
{
parent::addExt(new Ext1());
parent::addExt(new Ext2());
}
public function __toString()
{
return $this->getName().', from: '.$this->getCountry();
}
}
$o = new Extender();
$o->setName("fabio");
$o->setCountry("brazil");
echo $o;
?>
don't hesitate to email me, happy coding! ;)
Adel Abula adel at magebinary dot com ¶
9 years ago
You can always place the universal variables and functions into the deepest class. Then call or rewrite it in child classes. See example.
<?php
class ClassA
{
public $var;
function __construct()
{
$this->setVar();
$this->getVar();
}
protected function getVar()
{
echo $this->var;
}
public function setVar()
{
$this->var = 1000;
}
}
class ClassAChildA extends ClassA
{
function __construct()
{
$this->setVar();
$this->getVar();
}
public function setVar()
{
$this->var = 1001;
}
}
class ClassAChildB extends ClassA
{
function __construct()
{
$this->var = 1002;
$this->getVar();
}
public function getVar()
{
echo $this->var;
}
}
//prints 1000
$parent = new ClassA();
//prints 1001
$childA = new ClassAChildA();
//prints 1002
$childB = new ClassAChildB();
?>
volte6 at nowhere dot com ¶
19 years ago
When declaring a class that relies upon another file ( because it extends the class defined in that file ), you should ALWAYS require_once() that file at the top.
This applies even when planning on looping through and including everything in the folder. Use require_once() in your loop, and at the top of the file that NEEDS the include.
geilt at esotech dot org ¶
12 years ago
I have found this the best way to grab a parents variables and such (that I love to set in the constructor, like standardized table names, etc.). I just call the parents constructor in the child constructor, then change variables inside the child constructor that needs to be customized for that class, or just leave it alone to make sure I understand I am using the parents constructor in the child constructor. (it's a good note and practice of mine.).
<?php
class MyParentClass
{
$this->table_one = "my_table";
$this->table_two = "variable_table";
}
class MyChildClass extends MyParentClass
{
public function construct()
{
parent::__construct();
$this->table_two = "newly_defined_table";
}
public function action()
{
$table1 = $this->table_one;
$table2 = $this->table_two;
return $table1 . " - " . $table2;
}
}
$myChildClass = new MyChildClass;
echo $myChildClass->action();
//Displays my_table - newly_defined_table
?>
mazsolt at yahoo dot com ¶
21 years ago
Just a simple example about inheritance:
<?php
class a1{
var $a=10;
function a1($a){
$this->a=$a;
}
}
class a2 extends a1{
var $x=11;
function a2($x,$y){
$this->x=$x;
parent::a1($y); // or a1::a1($y) or $this->a1($y)
}
}
class a3 extends a2{
var $q=999;
}
$x=new a3(99,9);
echo $x->a,"<br>",$x->x,"<br> ",$x->q;
?>
The output will be:
9
99
999
tomnezvigin at comcast dot net ¶
19 years ago
This may seem obvious, but check this scenario. You have a class folder:
+ class
--classA.php
--classB.php
--classC.php
--mainClass.php
Here... classA, classB, classC all extend the mainClass.
If you try to create a function that automatically includes all of the classes in a folder, normally, they are included alphabetically.
When you try to instantiate classC, for example, you will get an error:
"Cannot inherit from undefined class mainClass"
EVEN IF you instantiate the mainClass before you instantiate all of the other classes.
In other words, make sure your primary class is included before all others.
Bash I. ¶
19 years ago
Here is a simple idea that I use when I need my abstract classes (the inherited classes) implemented before my functional classes.
<?php
$_CLASSES = array_merge (
glob ("classes/*/*.abstract.php"),
glob ("classes/*/*.class.php")
);
foreach ($_CLASSES AS $_CLASS) {
require ($_CLASS);
}
?>
efredin at redtempest dot com ¶
20 years ago
It is possible to override a method innherited from a parent class by simply re-defining the method (for those of us who enjoy using abstract classes).
<?php
class A
{
var $foo;
function A()
{
$this->foo = "asdf";
}
function bar()
{
echo $this->foo." : Running in A";
}
}
class B extends A
{
function bar()
{
echo $this->foo." : Running in B";
}
}
$myClass = new B;
$myClass->bar();
?>
datahell at elxis dot org ¶
13 years ago
If you have a class that extends an other and both classes have a function with the same name then visibility to both classes should be the same, else you will have a fatal error.
An other interested part in the example bellow is that if visibility of the showName method is private then $this->name() will execute the showName method on class test. If it is public or protected it will execute method showName on class extendTest.
<?php
class test {
public function __construct() {
}
public function name() {
$this->xname('John');
}
private function showName($name) {
echo 'my name in test is '.$name;
}
}
class extendTest extends test {
public function __construct() {
parent::__construct();
}
private function showName($name) {
echo 'my name in extendTest is '.$name;
}
}
$test = new extendTest();
$test->name();
?>
result: my name in test is John
If we change visibility of the showName method to public or protected then the result of the above will be:
my name in extendTest is John
calimero at creatixnet dot com ¶
21 years ago
Just a quick note to make things more clear : while multiple inheritance is not allowed, several levels of single inheritance ARE ALLOWED indeed. Just test this example :
<?php
class A {
var $name='A';
function disp() {
echo $this->name;
}
}
class B extends A {
var $name='B';
}
class C extends B {
var $name='C';
}
$truc = new C() ;
$truc->disp(); // Will output C
?>
This is especially important to keep in mind while building a huge object hierarchy. for example :
+GenericObject
->+ Person
->->Employee
->+Computer
->->+WorkStation
->->-> PPC
->->-> Intel
->->+Server
->->->LDAPServer
->->->IntranetWebServer
.. and so on. Multiple level hierarchy relationship are possible in a tree-like structure (each child has one and only one parent, except for the root object).
alan hogan ¶
18 years ago
Just a note: It is possible to have a class inherit from multiple other classes, but only in a one-at-a-time linear hierarchy.
So this works, and C gets A and B functions:
<?php
class A {
public function af() { print 'a';}
public function bark() {print ' arf!';}
}
class B extends A {
public function bf() { print 'b';}
}
class C extends B {
public function cf() { print 'c';}
public function bark() {print ' ahem...'; parent::bark();}
}
$c = new C;
$c->af(); $c->bf(); $c->cf();
print "<br />";
$c->bark();
/**results:**/
//abc
//ahem... arf!
?>
This does NOT work:
<?php
class A {
public function af() { print 'a';}
public function bark() {print ' arf!';}
}
class B {
public function bf() { print 'b';}
}
class C extends B, A /*illegal*/ {
public function cf() { print 'c';}
public function bark() {print ' ahem...'; parent::bark();}
}
$c = new C;
$c->af(); $c->bf(); $c->cf();
print "<br />";
$c->bark();
//Parse Error
?>
admin at dmsoft-wy dot com ¶
14 years ago
You don't need to include_once the parent class if it's already declared. You do have to load the parent first and then the child.
vittorio dot zamparella at gmail dot com ¶
9 years ago
extending a PHP class with an included file "delays" the compilation:
<?php
require 'MySuperclass.php';
var_dump(class_exists('MyFirstClass')); // FALSE
class MyFirstClass extends MySuperclass {}
// the "compilation" of this is delayed till execution
var_dump(class_exists('MyFirstClass')); // TRUE
class AnotherSuperclass { /* nothing */}
var_dump(class_exists('MySecondClass'));
// returns TRUE even if it's called "before" definition
class MySecondClass extends AnotherSuperclass {}
?>
see also the examples at http://php.net/manual/en/function.class-exists.php
discoverd this when autoinitializing some static classes
"inerte" is my hotmail.com username ¶
22 years ago
[Editor's note: For an alternative to multiple inheritance, see the dynamic binding via object aggregation in the corresponding section of the manual.]
Multiple Inheritance is not supported but it is easy to emulate it:
<?php
class multipleInheritance
{
function callClass($class_to_call)
{
return new $class_to_call();
}
}
class A
{
function insideA()
{
echo "I'm inside A!<br />";
}
}
class B
{
function insideB()
{
echo "I'm inside B!<br />";
}
}
class C extends multipleInheritance
{
function insideC()
{
$a = parent::callClass('A');
$a->insideA();
$b = parent::callClass('B');
$b->insideB();
}
}
$c = new C();
$c->insideC();
?>
---
This will succesfully echo:
I'm inside A!
I'm inside B!
griffon9 at hotmail dot com ¶
22 years ago
Just to clarify something about inheritance. The following code :
<?php
class a
{
function call()
{
$this->toto();
}
function toto()
{
echo('Toto of A');
}
}
class b extends a
{
function toto()
{
echo('Toto of B');
}
}
$b=new b;
$b->call();
?>
...will correctly display "toto of B" (that is, the function declared in the parent is correctly calling the redefined function in the child)
php_AT_undeen_DOT_com ¶
22 years ago
if the class B that extends class A does not have a constuctor function (i.e. a function named B), then the constructor function of A will be used instead, you don't need to make a constructor in B just to call the constructor of A.
For example:
<?php
class A
{
function A()
{
echo "HEY! I'm A!\n";
}
}
class B extends A
{
}
$b = new B();
?>
produces the output:
HEY! I'm A!
Frank ¶
14 years ago
Just a quick example of how PHP will handle a parent calling a function named in both the parent and the child class. You would think it might use the function the way it is defined in the parent, but it does use the function that is defined in the child.
<?php
class One{
function showOne(){
echo 'Function One prints';
}
function hitFunction(){
$this->showOne();
}
}
class Two extends One{
function showOne(){
echo 'Function Two prints';
}
}
$thistwo = new Two;
$thistwo->hitFunction(); //prints "Function Two prints"
?>
Msquared ¶
20 years ago
Multiple inheritence is often more trouble than it's worth. For example, you have a class foo that inherits from both class bar and class baz. Classes bar and baz both have a fubar() method. When you create a foo object and call its fubar() method, which fubar() method is called: bar's or baz's?
It seems to me that using aggregate to glue one class's methods and data to another object is a bit like Ruby's fixins, but I could be wrong...
[[Editor's note:
The aggregate_* functions have been dropped, as of PHP 5
-S
]]
schultz at rancon dot de ¶
22 years ago
This prints out 'ab'. No need to create a new instance of a, therefor both methods still exists with same name.
<?php
class a {
function samename(){
echo 'a';
}
}
class b extends a{
function samename(){
echo 'b';
}
function b(){
a::samename();
b::samename();
}
}
$test_obj = new b();
?>
