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 --->+---------+
//
// +---------+
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="qux"
?>
기초
클래스
클래스 정의는 class 키워드로 시작합니다. 그 다음으로 클래스명, 마지막으로 클래스의 프로퍼티 정의와 메서드 정의를 감싸는 curly brace 쌍이 뒤따릅니다.
클래스명은 유효한 이름 이어야 하며, PHP에 예약된 이름reserved word도 아니어야 합니다. 유효한 클래스명은 하나의 글자나 언더스코어로 시작하고, 이어 여러개의 문자,숫자,언더스코어가 뒤따라야 합니다. 정규표현식으로 표현하자면 다음과 같이 표현합니다.: ^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$.
클래스는 클래스만의 상수, 변수 를 가질 수(이른바 "프로퍼티") 있습니다. 그리고, 함수를 가질 수(이른바 "메서드") 있습니다.
Example #1 간단한 클래스 정의
<?php
class SimpleClass
{
// 프로퍼티 정의
public $var = 'a default value';
// 메서드 정의
public function displayVar() {
echo $this->var;
}
}
?>
$this 는 이 클래스가 인스턴스화 되었을 때에만 사용될수 있습니다. $this 는 객체에 대한 참조입니다.(보통은 해당 메서드를 소유하는 객체이지만, 다른 객체일 수도 있습니다. 메서드가 다른 객체내부에서 정적으로 불려지는 경우가 그러합니다.)
Example #2 $this 에 대한 몇가지 예제
<?php
class A
{
function foo()
{
if (isset($this)) {
echo '$this is defined (';
echo get_class($this);
echo ")\n";
} else {
echo "\$this is not defined.\n";
}
}
}
class B
{
function bar()
{
// Note: 다음 라인은 E_STRICT 가 활성화 되었을 경우 warning을 발생시킵니다.
A::foo();
}
}
$a = new A();
$a->foo();
// Note: 다음 라인은 E_STRICT 가 활성화 되었을 경우 warning을 발생시킵니다.
A::foo();
$b = new B();
$b->bar();
// Note: 다음 라인은 E_STRICT 가 활성화 되었을 경우 warning을 발생시킵니다.
B::bar();
?>
위 예제의 출력:
$this is defined (A) $this is not defined. $this is defined (B) $this is not defined.
new
클래스의 인스턴스를 생성하기 위해서는 new 키워드를 사용해야 합니다. 생성자 가 정의되어 있는한 객체가 생성될것이고, 에러가 발생하면 예외를 발생시킬 것입니다. 클래스가 인스턴스화 되기 위해서는 미리 정의 되어 있어야 합니다.(필수가 되는 경우가 있습니다.)
만약에 클래스명을 포함하는 문자열 변수가 있다면 new 와 함께 사용할수 있습니다. 해당 클래스의 새로운 인스턴스가 생성될 것입니다. 만약에 클래스가 네임스페이스내에 속한다면, 완전한 이름을 사용해야 합니다.
Example #3 인스턴스 생성
<?php
$instance = new SimpleClass();
// 변수를 사용해서 가능합니다.:
$className = 'SimpleClass';
$instance = new $className(); // new SimpleClass()
?>
클래스 컨텍스트 내에서는 new self 와 new parent 를 통해 새로운 객체를 생성하는게 가능합니다.
이미 생성된 클래스의 인스턴스를 새 변수에 할당할때, 새 변수는 이미 할당된 동일한 객체의 인스턴스에 접근하게 될것입니다. 이것은 함수로 인스턴스를 넘길때에도 동일하게 동작합니다. 이미 생성된 객체를 복사하는것은 복제 를 통해 가능할 것입니다.
Example #4 객체 할당
<?php
$instance = new SimpleClass();
$assigned = $instance;
$reference =& $instance;
$instance->var = '$assigned will have this value';
$instance = null; // $instance and $reference become null
var_dump($instance);
var_dump($reference);
var_dump($assigned);
?>
위 예제의 출력:
NULL
NULL
object(SimpleClass)#1 (1) {
["var"]=>
string(30) "$assigned will have this value"
}
PHP 5.3.0 에서는 객체 인스턴스를 생성하는 다른 방법이 소개 되었습니다.:
Example #5 새로운 객체 생성
<?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);
?>
위 예제의 출력:
bool(true) bool(true) bool(true)
확장
클래스는 다른 클래스로부터 메서드와 프로퍼티 를 extends 키워드를 사용하여 선언함으로써 상속받을수 있습니다. 다수의 클래스로부터 상속받는것은 가능하지 않습니다; 단 하나의 기본 클래스로부터 상속을 받을 수 있습니다.
상속된 메서드와 프로퍼티는 부모클래스의 정의된 이름과 똑같은 이름으로 재정의 될수 있습니다. 하지만, 부모 클래스의 메서드가 final 로 정의 되어 있다면, 재정의 될수 없습니다. 재정의된 원래의 메서드나 정적 프로퍼티는 parent:: 로 접근할 수 있습니다.
메서드를 재정의할때, 파라미터는 원래의 메서드와 동일해야 하며, 그렇지 않을 경우 E_STRICT 레벨 에러를 발생시킬 것입니다.
생성자는 이와 다르게, 파라미터가 달라도 재정의 가능합니다.
Example #6 간단한 클래스 상속
<?php
class ExtendClass extends SimpleClass
{
// Redefine the parent method
function displayVar()
{
echo "Extending class\n";
parent::displayVar();
}
}
$extended = new ExtendClass();
$extended->displayVar();
?>
위 예제의 출력:
Extending class a default value
::class
PHP 5.5 이후에는 class 키워드로 클래스의 이름을 알아낼수 있습니다. ClassName 의 완전한 클래스명을 ClassName::class 를 이용해서 얻어낼 수 있습니다. 특히, 네임스페이스를 갖춘 클래스일 경우 유용합니다.
Example #7 클래스명 확인
<?php
namespace NS {
class ClassName {
}
echo ClassName::class;
}
?>
위 예제의 출력:
NS\ClassName
add a note
User Contributed Notes 11 notes
aaron at thatone dot com ¶
17 years ago
kStarbe at gmail point com ¶
7 years ago
You start using :: in second example although the static concept has not been explained. This is not easy to discover when you are starting from the basics.
Doug ¶
14 years ago
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
*/
?>
Hayley Watson ¶
6 years ago
Class names are case-insensitive:
<?php
class Foo{}
class foo{} //Fatal error.
?>
Any casing can be used to refer to the class
<?php
class bAr{}
$t = new Bar();
$u = new bar();
echo ($t instanceof $u) ? "true" : "false"; // "true"
echo ($t instanceof BAR) ? "true" : "false"; // "true"
echo is_a($u, 'baR') ? "true" : "false"; // "true"
?>
But the case used when the class was defined is preserved as "canonical":
<?php
echo get_class($t); // "bAr"
?>
And, as always, "case-insensitivity" only applies to ASCII.
<?php
class пасха{}
class Пасха{} // valid
$p = new ПАСХА(); // Uncaught warning.
?>
wbcarts at juno dot com ¶
16 years ago
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!
moty66 at gmail dot com ¶
15 years ago
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
$ob->getFoo(); // output: I am a new foo
?>
Regards
Motaz Abuthiab
pawel dot zimnowodzki at gmail dot com ¶
2 years ago
Although there is no null-safe operator for not existed array keys I found workaround for it: ($array['not_existed_key'] ?? null)?->methodName()
Notes on stdClass ¶
15 years ago
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)
johannes dot kingma at gmail dot com ¶
3 years ago
BEWARE!
Like Hayley Watson pointed out class names are not case sensitive.
<?php
class Foo{}
class foo{} // Fatal error: Cannot declare class foo, because the name is already in use
?>
As well as
<?php
class BAR{}
$bar = new Bar();
echo get_class($bar);
?>
Is perfectly fine and will return 'BAR'.
This has implications on autoloading classes though. The standard spl_autoload function will strtolower the class name to cope with case in-sensitiveness and thus the class BAR can only be found if the file name is bar.php (or another variety if an extension was registered with spl_autoload_extensions(); ) not BAR.php for a case sensitive file and operating system like linux. Windows file system is case sensitive but the OS is not and there for autoloading BAR.php will work.
Anonymous ¶
6 years ago
At first I was also confused by the assignment vs referencing but here's how I was finally able to get my head around it. This is another example which is somewhat similar to one of the comments but can be helpful to those who did not understand the first example. Imagine object instances as rooms where you can store and manipulate your properties and functions. The variable that contains the object simply holds 'a key' to this room and thus access to the object. When you assign this variable to another new variable, what you are doing is you're making a copy of the key and giving it to this new variable. That means these two variable now have access to the same 'room' (object) and can thus get in and manipulate the values. However, when you create a reference, what you doing is you're making the variables SHARE the same key. They both have access to the room. If one of the variable is given a new key, then the key that they are sharing is replaced and they now share a new different key. This does not affect the other variable with a copy of the old key...that variable still has access to the first room
Jeffrey ¶
16 years ago
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.
