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 19-1. $this variable in object-oriented language
<?php
class A
{
function foo()
{
if (isset($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();
?>
|
The above example will output: $this is defined (a)
$this is not defined.
$this is defined (b)
$this is not defined. |
|
Example 19-2. Simple Class definition
<?php
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 19-3. Class members' default value
<?php
class SimpleClass
{
// invalid member declarations:
public $var1 = 'hello '.'world';
public $var2 = <<<EOD
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.
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 19-4. Creating an instance
<?php
$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 19-5. Object Assignment
<?php
$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"
} |
|
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 19-6. Simple Class Inherintance
<?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 |
|
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