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 --->+---------+
//
// +---------+
// $assignment -->|(handle1)----+
// +---------+ |
// |
// v
// Object(1):foo="qux"
?>
The Basics
class
Basic class definitions begin with the keyword class, followed by a class name, followed by a pair of curly braces which enclose the definitions of the properties and methods belonging to the class.
The class name can be any valid label, provided it is not a
PHP reserved word. A valid class
name starts with a letter or underscore, followed by any number of
letters, numbers, or underscores. As a regular expression, it
would be expressed thus:
^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$.
A class may contain its own constants, variables (called "properties"), and functions (called "methods").
Example #1 Simple Class definition
<?php
class SimpleClass
{
// property declaration
public $var = 'a default value';
// method declaration
public function displayVar() {
echo $this->var;
}
}
?>
The pseudo-variable $this is available when a method is called from within an object context. $this is a reference to the calling object (usually the object to which the method belongs, but possibly another object, if the method is called statically from the context of a secondary object). As of PHP 7.0.0 calling a non-static method statically from an incompatible context results in $this being undefined inside the method. Calling a non-static method statically from an incompatible context has been deprecated as of PHP 5.6.0. As of PHP 7.0.0 calling a non-static method statically has been generally deprecated (even if called from a compatible context). Before PHP 5.6.0 such calls already triggered a strict notice.
Example #2 Some examples of the $this pseudo-variable
We're assuming that error_reporting is disabled for this example; otherwise the following code would trigger deprecated and strict notices, respectively, depending on the PHP version.
<?php
class A
{
function foo()
{
if (isset($this)) {
echo '$this is defined (';
echo get_class($this);
echo ")\n";
} else {
echo "\$this is not defined.\n";
}
}
}
class B
{
function bar()
{
A::foo();
}
}
$a = new A();
$a->foo();
A::foo();
$b = new B();
$b->bar();
B::bar();
?>
Output of the above example in PHP 5:
$this is defined (A) $this is not defined. $this is defined (B) $this is not defined.
Output of the above example in PHP 7:
$this is defined (A) $this is not defined. $this is not defined. $this is not defined.
new
To create an instance of a class, the new keyword must be used. An object will always be created unless the object has a constructor defined that throws an exception on error. Classes should be defined before instantiation (and in some cases this is a requirement).
If a string containing the name of a class is used with new, a new instance of that class will be created. If the class is in a namespace, its fully qualified name must be used when doing this.
Note:
If there are no arguments to be passed to the class's constructor, parentheses after the class name may be omitted.
Example #3 Creating an instance
<?php
$instance = new SimpleClass();
// This can also be done with a variable:
$className = 'SimpleClass';
$instance = new $className(); // new SimpleClass()
?>
In the class context, it is possible to create a new object by new self and new parent.
When assigning an already created instance of a class to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function. A copy of an already created object can be made by cloning it.
Example #4 Object Assignment
<?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);
?>
The above example will output:
NULL
NULL
object(SimpleClass)#1 (1) {
["var"]=>
string(30) "$assigned will have this value"
}
PHP 5.3.0 introduced a couple of new ways to create instances of an object:
Example #5 Creating new objects
<?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);
?>
The above example will output:
bool(true) bool(true) bool(true)
PHP 5.4.0 introduced the possibility to access a member of a newly created object in a single expression:
Example #6 Access member of newly created object
<?php
echo (new DateTime())->format('Y');
?>
The above example will output something similar to:
2016
Properties and methods
Class properties and methods live in separate "namespaces", so it is possible to have a property and a method with the same name. Referring to both a property and a method has the same notation, and whether a property will be accessed or a method will be called, solely depends on the context, i.e. whether the usage is a variable access or a function call.
Example #7 Property access vs. method call
<?php
class Foo
{
public $bar = 'property';
public function bar() {
return 'method';
}
}
$obj = new Foo();
echo $obj->bar, PHP_EOL, $obj->bar(), PHP_EOL;
The above example will output:
property method
That means that calling an anonymous function which has been assigned to a property is not directly possible. Instead the property has to be assigned to a variable first, for instance. As of PHP 7.0.0 it is possible to call such a property directly by enclosing it in parentheses.
Example #8 Calling an anonymous function stored in a property
<?php
class Foo
{
public $bar;
public function __construct() {
$this->bar = function() {
return 42;
};
}
}
$obj = new Foo();
// as of PHP 5.3.0:
$func = $obj->bar;
echo $func(), PHP_EOL;
// alternatively, as of PHP 7.0.0:
echo ($obj->bar)(), PHP_EOL;
The above example will output:
42
extends
A class can inherit the methods and properties of another class by using the keyword extends in the class declaration. It is not possible to extend multiple classes; a class can only inherit from one base class.
The inherited methods and properties can be overridden by redeclaring them with the same name defined in the parent class. However, if the parent class has defined a method as final, that method may not be overridden. It is possible to access the overridden methods or static properties by referencing them with parent::.
When overriding methods, the parameter signature should remain the same or
PHP will generate an E_STRICT level error. This does
not apply to the constructor, which allows overriding with different
parameters.
Example #9 Simple Class Inheritance
<?php
class ExtendClass extends SimpleClass
{
// Redefine the parent method
function displayVar()
{
echo "Extending class\n";
parent::displayVar();
}
}
$extended = new ExtendClass();
$extended->displayVar();
?>
The above example will output:
Extending class a default value
::class
Since PHP 5.5, the class keyword is also used for class name resolution. You can get a string containing the fully qualified name of the ClassName class by using ClassName::class. This is particularly useful with namespaced classes.
Example #10 Class name resolution
<?php
namespace NS {
class ClassName {
}
echo ClassName::class;
}
?>
The above example will output:
NS\ClassName
Note:
The class name resolution using ::class is a compile time transformation. That means at the time the class name string is created no autoloading has happened yet. As a consequence, class names are expanded even if the class does not exist. No error is issued in that case.
add a note
User Contributed Notes 19 notes
aaron at thatone dot com ¶
12 years ago
kStarbe at gmail point com ¶
2 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 ¶
9 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
*/
?>
Anonymous ¶
1 year ago
Note that $this cannot be reassigned, because most instance functions would not work. So this code throws a fatal error :
<?php
class Foo {
function reassign() {
// This line triggers an E_ERROR
$this = new Foo();
}
}
$example = new Foo();
$example->reassign();
?>
Hayley Watson ¶
2 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 ¶
11 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!
Anonymous ¶
1 year 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
Notes on stdClass ¶
10 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)
Manish Gupta ¶
8 years ago
Some thing that may be obvious to the seasoned PHP programmer, but may surprise someone coming over from C++:
<?php
class Foo
{
$bar = 'Hi There';
public function Print(){
echo $bar;
}
}
?>
Gives an error saying Print used undefined variable. One has to explicitly use (notice the use of <?php $this->bar ?>):
<?php
class Foo
{
$bar = 'Hi There';
public function Print(){
echo this->$bar;
}
}
?>
<?php echo $this->bar; ?> refers to the class member, while using $bar means using an uninitialized variable in the local context of the member function.
Anonymous ¶
3 years ago
Understanding what does $this exactly do:
<?php
class toop {
public $i = "1234";
public function setv($a) {
$this->i = $a;
}
public function returnthis() {
return $this;
}
}
$ob = new toop();
$ob1 = new toop();
$obthis = $ob->returnthis();
echo $ob->i."\n";
$ob->setv("\$ob set");
echo $ob->i."\n";
$obthis->setv("\$obthis set");
echo $ob->i."\n";
$ob1->setv('$ob1 set');
echo $ob->i."\n";
?>
This will output:
1234
$ob set
$obthis set
$obthis set
moty66 at gmail dot com ¶
10 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
Anonymous ¶
2 years ago
Perhaps the note about ::class expansion during compile time is not entirely clear. Here's an example to demonstrate what happens when using ::class on non-existent classes:
<?php
var_dump(ExampleAbc::class);
var_dump(NS\Service\ExampleAbc::class);
// Outputs:
// string(10) "ExampleAbc"
// string(21) "NS\Service\ExampleAbc"
// No error is issued.
?>
Jeffrey ¶
11 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.
the_french_cow at hotmail dot com ¶
10 years ago
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.
Marcus ¶
9 years ago
Here's another simple example.
<?php
// PHP 5
// class definition
class Bear {
// define properties
public $name;
public $weight;
public $age;
public $sex;
public $colour;
// constructor
public function __construct() {
$this->age = 0;
$this->weight = 100;
}
// define methods
public function eat($units) {
echo $this->name." is eating ".$units." units of food... ";
$this->weight += $units;
}
public function run() {
echo $this->name." is running... ";
}
public function kill() {
echo $this->name." is killing prey... ";
}
public function sleep() {
echo $this->name." is sleeping... ";
}
}
// extended class definition
class PolarBear extends Bear {
// constructor
public function __construct() {
parent::__construct();
$this->colour = "white";
$this->weight = 600;
}
// define methods
public function swim() {
echo $this->name." is swimming... ";
}
}
?>
info at youwanttoremovethisvakantiebaas dot nl ¶
11 years ago
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 :)
nagabhushan1995 at gmail dot com ¶
2 years ago
Object handles in PHP are kind of Java References (not C++ references : references are aliases in C++).
Hayley Watson ¶
2 years ago
Unless the function's parameter is marked as pass by reference using "&", arguments to it are passed by value. This goes for objects same as everything else. The fact is every object has its own unique identity, and the "value" of an object IS the object itself. And not, for example, anything to do with the values of any of its properties: modifying an object's properties doesn't change the object's identity (and hence its value) any more than your identity changes when you take your socks off. So when an object is passed (by value) to a function, and its properties are modified, THAT object's properties get modified.
The "&" modifier only matters if a new value is assigned to the parameter variable. If the modifier is used, then whatever was passed in the parameter is also replaced, if it's not, then it isn't. Again, this behaviour is the same whether it's an object or something else.
nagabhushan1995 at gmail dot com ¶
2 years ago
Object handles in PHP are kind of Java References (not C++ references : references are aliases in C++).
