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Einfache Klassendefinitionen beginnen mit dem Schlüsselwort class, gefolgt von einem Klassennamen, gefolgt von einem Paar geschweifter Klammern, die die Definitionen der Eigenschaften und Methoden der Klasse enthalten.
Der Klassenname kann jeder gültige Bezeichner sein, der kein von PHP reserviertes Wort ist. Ein gültiger Klassenname beginnt mit einem Buchstaben oder einem Unterstrich, gefolgt von einer beliebigen Anzahl von Buchstaben, Ziffern oder Unterstrichen; als regulärer Ausdruck formuliert: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*.
Eine Klasse darf aus ihren eigenen Konstanten, Variablen ("Eigenschaften" genannt) und Funktionen ("Methoden" genannt) bestehen.
Beispiel #1 Definition einer einfachen Klasse
<?php
class SimpleClass
{
// Deklaration einer Eigenschaft
public $var = 'ein Standardwert';
// Deklaration einer Methode
public function displayVar() {
echo $this->var;
}
}
?>
Die Pseudovariable $this ist verfügbar, falls eine Methode aus einem Objektkontext heraus aufgerufen wird. $this ist eine Referenz auf das aufrufende Objekt (üblicherweise das Objekt, zu dem die Methode gehört, aber möglicherweise ein anderes Objekt, falls die Methode statisch aus dem Kontext eines zusätzlichen Objektes aufgerufen wird).
Beispiel #2 Einige Beispiele für die $this-Pseudovariable
<?php
class A
{
function foo()
{
if (isset($this)) {
echo '$this ist definiert (';
echo get_class($this);
echo ")\n";
} else {
echo "\$this ist nicht definiert.\n";
}
}
}
class B
{
function bar()
{
// Hinweis: die folgende Zeile führt zu einer Warnung, wenn
// E_STRICT aktiviert ist
A::foo();
}
}
$a = new A();
$a->foo();
// Hinweis: die folgende Zeile führt zu einer Warnung, wenn E_STRICT aktiviert ist
A::foo();
$b = new B();
$b->bar();
// Hinweis: die folgende Zeile führt zu einer Warnung, wenn E_STRICT aktiviert ist
B::bar();
?>
Das oben gezeigte Beispiel erzeugt folgende Ausgabe:
$this ist definiert (A) $this ist nicht definiert. $this ist definiert (B) $this ist nicht definiert.
Um eine Instanz einer Klasse zu erzeugen, muss das new-Schlüsselwort verwendet werden. Ein Objekt wird immer erzeugt, außer wenn das Objekt einen definierten Konstruktor besitzt, der aufgrund eines Fehlers eine Exception wirft. Klassen sollten vor ihrer Instantiierung definiert werden (in manchen Fällen ist dies eine Notwendigkeit).
Wenn ein string mit dem Namen einer Klasse mit new genutzt wird, wird eine neue Instanz dieser Klasse erzeugt. Falls sich die Klasse in einem Namensraum befindet, muss der vollqualifizierte Name hierfür genutzt werden.
Beispiel #3 Eine Instanz erzeugen
<?php
$instanz = new SimpleClass();
// dies ist auch mit einer Variablen möglich:
$klassenname = 'Foo';
$instanz = new $klassenname(); // Foo()
?>
Im Kontext einer Klasse ist es möglich, neue Objekte mit new self und new parent anzulegen.
Wenn man eine bereits erzeugte Instanz einer Klasse einer neuen Variablen zuweist, wird die neue Variable auf die selbe Instanz zugreifen wie das Objekt, das zugewiesen wurde. Dieses Verhalten ist das selbe, wenn man Instanzen an Funktionen übergibt. Eine Kopie eines bereits erzeugten Objektes erhält man, indem man es klont.
Beispiel #4 Objektzuweisung
<?php
$instanz = new SimpleClass();
$zugewiesen = $instanz;
$referenz =& $instanz;
$instanz->var = '$zugewiesen wird diesen Wert haben';
$instanz = null; // $instanz und $referenz werden null
var_dump($instanz);
var_dump($referenz);
var_dump($zugewiesen);
?>
Das oben gezeigte Beispiel erzeugt folgende Ausgabe:
NULL
NULL
object(SimpleClass)#1 (1) {
["var"]=>
string(30) "$zugewiesen wird diesen Wert haben"
}
Mit PHP 5.3.0 wurden neue Möglichkeiten zur Erzeugung von Instanzen eines Objekts eingeführt:
Beispiel #5 Erzeugen neuer Objekte
<?php
class Test
{
static public function getNew()
{
return new static;
}
}
class Child extends Test
{}
$obj1 = new Test();
$obj2 = new $obj1;
var_dump($obj1 !== $obj2);
$obj3 = Test::getNew();
var_dump($obj3 instanceof Test);
$obj4 = Child::getNew();
var_dump($obj4 instanceof Child);
?>
Das oben gezeigte Beispiel erzeugt folgende Ausgabe:
bool(true) bool(true) bool(true)
Eine Klasse kann Methoden und Eigenschaften einer anderen Klasse erben, indem man das extends-Schlüsselwort in der Deklaration benutzt. Es ist nicht möglich, mehrere Klassen zu erweitern; eine Klasse kann nur eine einzige Basisklasse beerben.
Die geerbten Methoden und Eigenschaften können durch eine Neudeklaration mit dem gleichen Namen wie in der Elternklasse überschrieben werden. Falls die Elternklasse eine Methode als final definiert hat, kann diese Methode nicht überschrieben werden. Es ist möglich, auf die überschriebenen Methoden oder statischen Eigenschaften zuzugreifen, wenn diese mittels parent:: referenziert werden.
Wenn Methoden überschrieben werden, sollte die Parametersignatur gleich bleiben; andernfalls wird PHP einen E_STRICT-Fehler generieren. Dies gilt nicht für den Konstruktor, der das Überschreiben mit unterschiedlichen Parameter erlaubt.
Beispiel #6 Einfache Vererbung
<?php
class ExtendClass extends SimpleClass
{
// Die Vatermethode überschreiben
function displayVar()
{
echo "Erweiternde Klasse\n";
parent::displayVar();
}
}
$extended = new ExtendClass();
$extended->displayVar();
?>
Das oben gezeigte Beispiel erzeugt folgende Ausgabe:
Erweiternde Klasse ein Vorgabewert
What is the difference between $this and self ?
Inside a class definition, $this refers to the current object, while self refers to the current class.
It is necessary to refer to a class element using self ,
and refer to an object element using $this .
Note also how an object variable must be preceded by a keyword in its definition.
The following example illustrates a few cases:
<?php
class Classy {
const STAT = 'S' ; // no dollar sign for constants (they are always static)
static $stat = 'Static' ;
public $publ = 'Public' ;
private $priv = 'Private' ;
protected $prot = 'Protected' ;
function __construct( ){ }
public function showMe( ){
print '<br> self::STAT: ' . self::STAT ; // refer to a (static) constant like this
print '<br> self::$stat: ' . self::$stat ; // static variable
print '<br>$this->stat: ' . $this->stat ; // legal, but not what you might think: empty result
print '<br>$this->publ: ' . $this->publ ; // refer to an object variable like this
print '<br>' ;
}
}
$me = new Classy( ) ;
$me->showMe( ) ;
/* Produces this output:
self::STAT: S
self::$stat: Static
$this->stat:
$this->publ: Public
*/
?>
method calling context aware. By this I mean it will get treated differently while being in a new statement compared to being in a regular call.
Example:
<?php
class Foo {
private $className = 'Bar';
public function make() {
return new $this->className();
}
public function callClassName() {
$this->className();
}
public function className() {
echo "foo\n";
}
};
class Bar {
public function hello() {
echo "bar\n";
}
};
$foo = new Foo();
$bar = $foo->make();
echo "expecting 'bar': ";
$bar->hello();
echo "expecting 'foo': ";
$foo->callClassName();
?>
even tough $this->className() is written two times in exactly the same way, the one contained in a new statement gets the className field and the other performs the actual method.
It is also simple to get or set a property with a name determined at runtime:
<?php
$e=new E();
$e->{"foo"} = 1; // using a runtime name
// is the same as doing:
// $e->foo = 1;
?>
I hope that this will help to understand how to work with static variables inside a class
<?php
class a {
public static $foo = 'I am foo';
public $bar = 'I am bar';
public static function getFoo() { echo self::$foo; }
public static function setFoo() { self::$foo = 'I am a new foo'; }
public function getBar() { echo $this->bar; }
}
$ob = new a();
a::getFoo(); // output: I am foo
$ob->getFoo(); // output: I am foo
//a::getBar(); // fatal error: using $this not in object context
$ob->getBar(); // output: I am bar
// If you keep $bar non static this will work
// but if bar was static, then var_dump($this->bar) will output null
// unset($ob);
a::setFoo(); // The same effect as if you called $ob->setFoo(); because $foo is static
$ob = new a(); // This will have no effects on $foo
$ob->getFoo(); // output: I am a new foo
?>
Regards
Motaz Abuthiab
ok this really basic but I always forget this. I always get an error like:
Fatal error: Call to a member function on a non-object
when i deal with oops
if it were me finding the error i'd search the internet for hours and then it would occur to me, I'm putting my class operator inside a function, but i would define the class in global file.
so like this:
test.php
<?
include(class.php);
$class = new newclassname;
function function1(){
$class->dofunc();
}
?>
you'll get some die errors and try and do this with function1,
function function1(){
newclassname::dofunc();
}
but if you're using $this inside your class then you'll get another error on non object
so basically, all you need to do is:
function function1(){
$class = new newclassname;
$class->dofunc();
}
or
function function1(){
global $class;
$class->dofunc();
}
i know it's simple, but it always gets me!
For those of us who are new to inheritance, private functions are not visible in an inherited class. Consider:
<?php
class A {
protected function func1() {
echo("I'm func1 in A!<br/>");
}
private function func2() {
echo("I'm func2 in A!<br/>");
}
}
class B extends A {
public function func3() {
echo("I'm func3 in B!<br/>");
$this->func1();
$this->func2(); // Call to private function from extended class results in a fatal error
}
}
$b = new B;
$b->func3(); // Ends in a fatal error
// OR
$b->func1(); // Call to protected function from outside world results in a fatal error
?>
If you want a function to be accessible in class B but not to the outside world, it must be declared as protected.
stdClass is the default PHP object. stdClass has no properties, methods or parent. It does not support magic methods, and implements no interfaces.
When you cast a scalar or array as Object, you get an instance of stdClass. You can use stdClass whenever you need a generic object instance.
<?php
// ways of creating stdClass instances
$x = new stdClass;
$y = (object) null; // same as above
$z = (object) 'a'; // creates property 'scalar' = 'a'
$a = (object) array('property1' => 1, 'property2' => 'b');
?>
stdClass is NOT a base class! PHP classes do not automatically inherit from any class. All classes are standalone, unless they explicitly extend another class. PHP differs from many object-oriented languages in this respect.
<?php
// CTest does not derive from stdClass
class CTest {
public $property1;
}
$t = new CTest;
var_dump($t instanceof stdClass); // false
var_dump(is_subclass_of($t, 'stdClass')); // false
echo get_class($t) . "\n"; // 'CTest'
echo get_parent_class($t) . "\n"; // false (no parent)
?>
You cannot define a class named 'stdClass' in your code. That name is already used by the system. You can define a class named 'Object'.
You could define a class that extends stdClass, but you would get no benefit, as stdClass does nothing.
(tested on PHP 5.2.8)
If you pass $this by reference and then assign a new value to it, it will not behave as you might expect as illustrated by this example:
<?php
class TestClass
{
public $data;
function setData(&$data) {
$this->data =& $data;
}
function hasData(&$data) {
$saved = $data;
$data = true;
$result = $this->data === true;
$data = $saved;
return $result;
}
function isDataOf(&$object) {
return $object->hasData($this);
}
}
$o1 = new TestClass;
$o2 = new TestClass;
$o1->setData($o2);
$o2->setData($o1);
var_dump($o1->hasData($o2)); // true as expected
var_dump($o2->hasData($o1)); // true as expected
var_dump($o1->isDataOf($o2)); // false even though $o1 is in fact the data of $o2
var_dump($o2->isDataOf($o1)); // false even though $o2 is in fact the data of $o1
?>
You can make this example work by replacing the hasData method with:
<?php
function hasData(&$data) {
return $data === $this->data;
}
?>
However, although I've not tested this, I've been told that Zend Engine 1, i.e. PHP 4, will choke on the === parameter when comparing recursing objects.
Unfortunately, Arpit's solution creates a new class and leaves the old class inaccessible. If you need access to members of the class you are in you'll be unable to get such access. This can be a huge problem.
However, there is a solution:
<?php
class MyClass {
public $message = 'Hello';
public function MyClassFunction() {
function innerFunction(&$this_thing, $message = null) {
$this_thing->message = (!is_null($message)) ? $message : $this_thing->message;
$this_thing->echo_something();
}
innerFunction($this); // echoes 'Hello'
innerFunction($this, '<br/>New Message'); // echoes '<br/>New Message'
}
public function echo_something() {
echo $this->message;
}
}
$class = new MyClass;
$class->MyClassFunction();
?>
By passing $this as a variable by reference, you can access members of the class and even update them. If you don't want to be able to update them, you can simply pass $this to the function but not as a reference.
//try this code if you define a new class inside an object method than we can refer to "$class->message"
//unset this instance doesn't affected the previous one
//it will not report a fatal error
<?php
class MyClass {
public $message = 'Hello';
public function MyClassFunction() {
function InnerFunction() {
$class = new MyClass;
print_r($class);
echo $class->message;
unset($class);//unset this doesn't affected the previous one or we can also use different name $classNew=new MyClass;
}
innerFunction();
}
}
$class = new MyClass;
$class->MyClassFunction();
?>
I think it's worth mentioning that if you define a function inside of an object method, that function cannot refer to "$this" - doing so will result in PHP reporting a fatal error:
Fatal error: Using $this when not in object context
<?php
class MyClass {
public $message = 'Hello';
public function MyClassFunction() {
function InnerFunction() {
echo $this->message; // Reports a fatal error
}
innerFunction();
}
}
$class = new MyClass;
$class->MyClassFunction();
?>
This issue cannot be solved by using the Scope Resolution Operator if you're trying to access a variable:
<?php
class MyClass {
public $message = 'Hello';
public function MyClassFunction() {
function InnerFunction() {
echo MyClass::message; // Reports a fatal error
}
innerFunction();
}
}
$class = new MyClass;
$class->MyClassFunction();
?>
Additionally, you can NOT create a public function to access that variable:
<?php
class MyClass {
public $message = 'Hello';
public function MyClassFunction() {
function InnerFunction() {
MyClass::echoSomething();
}
innerFunction();
}
public function echoSomething() {
echo $this->message; // Reports a fatal error
}
}
$class = new MyClass;
$class->MyClassFunction();
?>
Note that in this last case, the error is generated on the line below echoSomething function declaration, not at MyClass::echoSomething();
However, it is worth noting that when called directly, echoSomething works fine:
<?php
class MyClass {
public $message = 'Hello';
public function MyClassFunction() {
function InnerFunction() {
MyClass::echoSomething();
}
innerFunction();
}
public function echoSomething() {
echo $this->message; // Echoes 'Hello'
}
}
$class = new MyClass;
$class->echoSomething();
?>
A simple approach to Multiple Inheritance
You can give yourself something approaching multiple inheritance with the following class:
<?php
class inheritance{
var $bases = array();
static function error_die( $errno, $errstr, $errfile, $errline ) {
$backtrace = debug_backtrace();
$detail = $backtrace[4];
var_dump( $backtrace );
echo '<b>Fatal Error</b>: '.$errstr.' of class <b>'.$detail["class"].'</b> in <b>'.$detail["file"].'</b> on line <b>'.$detail["line"].'</b><br/>';
die();
}
private function fatal( $text ) {
set_error_handler( array( 'inheritance', 'error_die' ) );
trigger_error( $text, E_USER_ERROR );
restore_error_handler();
}
function __call( $name, $args ) {
if( $this->bases )
foreach( $this->bases as $base )
if( method_exists( $base, $name ) )
return $base->$name( $args );
$this->fatal( "Call to undefined method <b>".$name."</b>" );
}
function __set( $name, $value ) {
if( $this->bases )
foreach( $this->bases as $base )
if( property_exists( $base, $name ) ) {
$base->$name = $value;
return;
}
}
function __get( $name ) {
if( $this->bases )
foreach( $this->bases as $base )
if( property_exists( $base, $name ) )
return $base->$name;
}
function __isset( $name ) {
if( $this->bases )
foreach( $this->bases as $base )
if( property_exists( $base, $name ) )
return isset( $base->$name );
}
function __unset( $name ) {
if( $this->bases )
foreach( $this->bases as $base )
if( property_exists( $base, $name ) ) {
unset( $base->$name );
return;
}
}
function inherits( $name, $args = '' ) {
return array_unshift( $this->bases, new $name( $args ) );
}
}
?>
Most of the qualities of multiple inheritance provided by this class are revealed by the following code:
<?php //test inheritance
class base0 {
public $base0var;
public $basevar;
function base0declare() {
echo 'I am base 0';
}
function basedeclare() {
self::base0declare() {
}
}
class base1 extends base0 { // simple linear inheritance here
public $base1var;
public $basevar;
function based1declare() {
echo 'I am base 1';
}
function basedeclare() {
self::base1declare()
}
}
class base2
public $base2var;
public $basevar;
function based2declare() {
echo 'I am base 2';
}
function basedeclare() {
self::base2declare
}
}
?>
Multiple inheritance is achieved by extending the inheritance class, and then in the __construct function placing calls to the "inherits" method of the inheritance class. Each call pushes an instance of the inherited class into an array var which functions as a LIFO stack. Using the magic methods, any failed method call, property set, get, isset or unset is intercepted by the inheritance base class which then attempts to resolve the reference. Object method name conflicts are resolved simply by the later inheritance masking the scope of the earlier inherited method. I recognize there are shortcomings to the approach I offer here, but it works for all my current multiple inheritance needs and offers simplicity and ease of understanding as benefits.
<?php
class base_test extends inheritance { // multiple inheritance
function __construct() {
$this->inherits( 'base1' );
$this->inherits( 'base2' );
}
} ?>
Here are some code fragments you can try out to test things.
<?php
$testobj = new base_test();
var_dump( $testobj );
$testobj->base2declare();
$testobj->base1declare();
$testobj->base0declare();
$testobj->basedeclare();
$testobj->base2var = 27;
echo $testobj->base2var;
?>
I'd be interested in hearing any comments.
Regarding object inheritance:
I hope this helps someone, it should help if you're new to OOPS
<?php
class A {
public $x = 'A';
public function foo() {
$b = new B;
$b->bar();
return $this->x;
}
}
class B extends A {
public function bar() {
$this->x = 'B';
}
}
$a = new A
echo $a->foo(); //A
?>
I was doing something similar to this (example is greatly simplified to show logic) and spent a long while trying to work out why I would always get 'A' and never get 'B'. Now, after a few weeks, I have revisited the problem and have worked out why:
The code 'new B' creates a new instance of class B. While class B extends class A, it is a new object and not an extension of the object created by 'new A'
The value of $x is set to 'B' within the object $b, but not in object $a.
If within A::foo(), one was to access $b->x then one would obtain the vale 'B', for example
<?php
class C {
public $x = 'C';
public function foo() {
$c = new C;
$c->bar();
$this->x = $c->$x
return $this->x;
}
}
class D extends C {
public function bar() {
$this->x = 'D';
}
}
$c = new C
echo $c->foo(); //D
?>
A PHP Class can be used for several things, but at the most basic level, you'll use classes to "organize and deal with like-minded data". Here's what I mean by "organizing like-minded data". First, start with unorganized data.
<?php
$customer_name;
$item_name;
$item_price;
$customer_address;
$item_qty;
$item_total;
?>
Now to organize the data into PHP classes:
<?php
class Customer {
$name; // same as $customer_name
$address; // same as $customer_address
}
class Item {
$name; // same as $item_name
$price; // same as $item_price
$qty; // same as $item_qty
$total; // same as $item_total
}
?>
Now here's what I mean by "dealing" with the data. Note: The data is already organized, so that in itself makes writing new functions extremely easy.
<?php
class Customer {
public $name, $address; // the data for this class...
// function to deal with user-input / validation
// function to build string for output
// function to write -> database
// function to read <- database
// etc, etc
}
class Item {
public $name, $price, $qty, $total; // the data for this class...
// function to calculate total
// function to format numbers
// function to deal with user-input / validation
// function to build string for output
// function to write -> database
// function to read <- database
// etc, etc
}
?>
Imagination that each function you write only calls the bits of data in that class. Some functions may access all the data, while other functions may only access one piece of data. If each function revolves around the data inside, then you have created a good class.
CLASSES and OBJECTS that represent the "Ideal World"
Wouldn't it be great to get the lawn mowed by saying $son->mowLawn()? Assuming the function mowLawn() is defined, and you have a son that doesn't throw errors, the lawn will be mowed.
In the following example; let objects of type Line3D measure their own length in 3-dimensional space. Why should I or PHP have to provide another method from outside this class to calculate length, when the class itself holds all the neccessary data and has the education to make the calculation for itself?
<?php
/*
* Point3D.php
*
* Represents one locaton or position in 3-dimensional space
* using an (x, y, z) coordinate system.
*/
class Point3D
{
public $x;
public $y;
public $z; // the x coordinate of this Point.
/*
* use the x and y variables inherited from Point.php.
*/
public function __construct($xCoord=0, $yCoord=0, $zCoord=0)
{
$this->x = $xCoord;
$this->y = $yCoord;
$this->z = $zCoord;
}
/*
* the (String) representation of this Point as "Point3D(x, y, z)".
*/
public function __toString()
{
return 'Point3D(x=' . $this->x . ', y=' . $this->y . ', z=' . $this->z . ')';
}
}
/*
* Line3D.php
*
* Represents one Line in 3-dimensional space using two Point3D objects.
*/
class Line3D
{
$start;
$end;
public function __construct($xCoord1=0, $yCoord1=0, $zCoord1=0, $xCoord2=1, $yCoord2=1, $zCoord2=1)
{
$this->start = new Point3D($xCoord1, $yCoord1, $zCoord1);
$this->end = new Point3D($xCoord2, $yCoord2, $zCoord2);
}
/*
* calculate the length of this Line in 3-dimensional space.
*/
public function getLength()
{
return sqrt(
pow($this->start->x - $this->end->x, 2) +
pow($this->start->y - $this->end->y, 2) +
pow($this->start->z - $this->end->z, 2)
);
}
/*
* The (String) representation of this Line as "Line3D[start, end, length]".
*/
public function __toString()
{
return 'Line3D[start=' . $this->start .
', end=' . $this->end .
', length=' . $this->getLength() . ']';
}
}
/*
* create and display objects of type Line3D.
*/
echo '<p>' . (new Line3D()) . "</p>\n";
echo '<p>' . (new Line3D(0, 0, 0, 100, 100, 0)) . "</p>\n";
echo '<p>' . (new Line3D(0, 0, 0, 100, 100, 100)) . "</p>\n";
?>
<-- The results look like this -->
Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=1, y=1, z=1), length=1.73205080757]
Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=100, y=100, z=0), length=141.421356237]
Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=100, y=100, z=100), length=173.205080757]
My absolute favorite thing about OOP is that "good" objects keep themselves in check. I mean really, it's the exact same thing in reality... like, if you hire a plumber to fix your kitchen sink, wouldn't you expect him to figure out the best plan of attack? Wouldn't he dislike the fact that you want to control the whole job? Wouldn't you expect him to not give you additional problems? And for god's sake, it is too much to ask that he cleans up before he leaves?
I say, design your classes well, so they can do their jobs uninterrupted... who like bad news? And, if your classes and objects are well defined, educated, and have all the necessary data to work on (like the examples above do), you won't have to micro-manage the whole program from outside of the class. In other words... create an object, and LET IT RIP!
@info -- 20-April
This is because you requested class "b" before defining it, not because you defined class "b" before "a". It doesn't make a difference which class you define first.
if you do this
<?php
$x = new b();
class b extends a {}
class a { }
?>
PHP will tell you "class b not found", because you've defined class b before a. However, the error tells you something different.... Got me a little confused :)
If you just want to create a new object that extends another object and you want to copy all variables from the father object, you may use this piece of code:
<?php
$father =& new father();
$father->a_var = "Hello World.";
$son = new son($event);
$son->say_hello();
class father {
public $a_var;
}
class son extends father {
public function __construct($father_class) {
foreach ($father_class as $variable=>$value) {
$this->$variable = $value;
}
}
public function say_hello() {
echo "Son says: ".$this->a_var;
}
}
?>
This outputs:
Son says: Hello World.
So you dont have to clone the entire object to get the contents of the variables from the father object.
I was confused at first about object assignment, because it's not quite the same as normal assignment or assignment by reference. But I think I've figured out what's going on.
First, think of variables in PHP as data slots. Each one is a name that points to a data slot that can hold a value that is one of the basic data types: a number, a string, a boolean, etc. When you create a reference, you are making a second name that points at the same data slot. When you assign one variable to another, you are copying the contents of one data slot to another data slot.
Now, the trick is that object instances are not like the basic data types. They cannot be held in the data slots directly. Instead, an object's "handle" goes in the data slot. This is an identifier that points at one particular instance of an obect. So, the object handle, although not directly visible to the programmer, is one of the basic datatypes.
What makes this tricky is that when you take a variable which holds an object handle, and you assign it to another variable, that other variable gets a copy of the same object handle. This means that both variables can change the state of the same object instance. But they are not references, so if one of the variables is assigned a new value, it does not affect the other variable.
<?php
// Assignment of an object
Class Object{
public $foo="bar";
};
$objectVar = new Object();
$reference =& $objectVar;
$assignment = $objectVar
//
// $objectVar --->+---------+
// |(handle1)----+
// $reference --->+---------+ |
// |
// +---------+ |
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="bar"
//
?>
$assignment has a different data slot from $objectVar, but its data slot holds a handle to the same object. This makes it behave in some ways like a reference. If you use the variable $objectVar to change the state of the Object instance, those changes also show up under $assignment, because it is pointing at that same Object instance.
<?php
$objectVar->foo = "qux";
print_r( $objectVar );
print_r( $reference );
print_r( $assignment );
//
// $objectVar --->+---------+
// |(handle1)----+
// $reference --->+---------+ |
// |
// +---------+ |
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="qux"
//
?>
But it is not exactly the same as a reference. If you null out $objectVar, you replace the handle in its data slot with NULL. This means that $reference, which points at the same data slot, will also be NULL. But $assignment, which is a different data slot, will still hold its copy of the handle to the Object instance, so it will not be NULL.
<?php
$objectVar = null;
print_r($objectVar);
print_r($reference);
print_r($assignment);
//
// $objectVar --->+---------+
// | NULL |
// $reference --->+---------+
//
// +---------+
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="qux"
?>
The following odd behavior happens in php version 5.1.4 (and presumably some other versions) that does not happen in php version 5.2.1 (and possibly other versions > 5.1.4).
<?php
$_SESSION['instance']=...;
$instance=new SomeClass;
?>
The second line will not only create the $instance object successfully, it will also modify the value of $_SESSION['instance']!
The workaround I arrived at, after trial and error, was to avoid using object names which match a $_SESSION array key.
This is not intended to be a bug report, since it was apparently fixed by version 5.2.1, so it's just a workaround suggestion.
If E_STRICT is enabled, the first example will generate the following error (and a few others akin to it):
Non-static method A::foo() should not be called statically on line 26
The example should have explicitly declared the methods foo() and bar() as static:
class A
{
static function foo()
{
...
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