Classes & Objects In php5

class
Every class definition begins with the keyword class, followed by a class name, which can be any name that isn't a reserved word in PHP. Followed by a pair of curly braces, which contains the definition of the classes members and methods. A 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 can be another object, if the method is called statically from the context of a secondary object). This is illustrated in the following examples: Example $this variable in object-oriented language

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();
?>

The above example will output:

$this is defined (a)
$this is not defined.
$this is defined (b)
$this is not defined.



Example . Simple Class definition

class SimpleClass
{
// member declaration
public $var = 'a default value';

// method declaration
public function displayVar() {
echo $this->var;
}
}
?>


The default value must be a constant expression, not (for example) a variable, a class member or a function call. Example . Class members' default value

class SimpleClass
{
// invalid member declarations:
public $var1 = 'hello '.'world';
public $var2 = <<hello world
EOD;
public $var3 = 1+2;
public $var4 = self::myStaticMethod();
public $var5 = $myVar;

// valid member declarations:
public $var6 = myConstant;
public $var7 = self::classConstant;
public $var8 = array(true, false);


}
?>



Note: There are some nice functions to handle classes and objects. You might want to take a look at the Class/Object Functions.

new
To create an instance of a class, a new object must be created and assigned to a variable. An object will always be assigned when creating a new object 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).

Example . Creating an instance

$instance = new SimpleClass();
?>


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 . Object Assignment

$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"
}


extends
A class can inherit methods and members of another class by using the extends keyword in the declaration. It is not possible to extend multiple classes, a class can only inherit one base class.

The inherited methods and members can be overridden, unless the parent class has defined a method as final, by redeclaring them within the same name defined in the parent class. It is possible to access the overridden methods or members by referencing them with parent::

Example . Simple Class Inheritance

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