picture of Luis Argerich
This arcticle introduces Object Oriented Programming (OOP) in PHP. I will show you how to code less and better by using some OOP concepts and PHP tricks. Good luck!
Concepts of object oriented programming: There're differences between authors, I can mention that a OOP language must have:
The encapsulation is done in php using classes:

<?php

class Something {
    
// In OOP classes are usually named starting with a cap letter.
    
var $x;

    function 
setX($v) {
        
// Methods start in lowercase then use lowercase to seprate
        // words in the method name example getValueOfArea()
        
$this->x=$v;
    }

    function 
getX() {
        return 
$this->x;
    }
}

?>
Of course you can use your own nomenclature, but having a standarized one is useful.
Data members are defined in php using a "var" declaration inside the class, data members have no type until they are assigned a value. A data member might be an integer, an array, an associative array or even an object. Methods are defined as functions inside the class, to access data members inside the methods you have to use $this->name, otherwise the variable is local to the method.
You create an object using the new operator:
$obj=new Something;
Then you can use member functions by:
$obj->setX(5);
$see=$obj->getX();
The setX member function assigns 5 to the x datamember in the object obj (not in the class), then getX returns its value: 5 in this case.
You can access the datamembers from the object reference using for example: $obj->x=6; however, this is not a very good OOP practice. I enforce you to set datamembers by defining methods to set them and access the datamembers by using retrieving methods. You'll be a good OOP programmer if you consider data members inaccesible and only use methods from the object handler. Unfortunately PHP doesn't have a way to declare a data member private so bad code is allowed.
Inheritance is easy in php using the extend keyword.

<?php

class Another extends Something {
    var 
$y;
    function 
setY($v) {
        
// Methods start in lowercase then use lowercase to seperate
        // words in the method name example getValueOfArea()
        
$this->y=$v;
    }

    function 
getY() {
        return 
$this->y;
    }
}

?>
Objects of the class "Another" now has all the data members and methods of the parent class (Something) plus its own data members and methods. You can use:
$obj2=new Something;
$obj2->setX(6);
$obj2->setY(7);
Multiple inheritance is not supported so you can't make a class extend two or more different classes.
You can override a method in the derived class by redefining it, if we redefine getX in "Another" we can't no longer access method getX in "Something". If you declare a data member in a derived class with the same name as a data member in a Base class the derived data member "hides" the base class data member when you access it.
You might define constructors in your classes, constructors are methods with the same name as the class and are called when you create an object of the class for example:

<?php

class Something {         
    var 
$x;

    function 
Something($y) {
        
$this->x=$y;
    }

    function 
setX($v) {     
        
$this->x=$v;          
    }

    function 
getX() {
        return 
$this->x;
    }
}

?>
So you can create an object by:
$obj=new Something(6);
And the constructor automatically asigns 6 to the data member x. Constructors and methods are normal php functions so you can use default arguments.
function Something($x="3",$y="5")
Then:
$obj=new Something();    // x=3 and y=5
$obj=new Something(8);   // x=8 and y=5
$obj=new Something(8,9);   // x=8 and y=9
Default arguments are used in the C++ way so you can't pass a value to Y and let X take the default value, arguments are asigned form left to right and when no more arguments are found if the function expected more they take the default values.
When an object of a derived class is created only its constructor is called, the constructor of the Parent class is not called, this is a gotcha of PHP because constructor chaining is a classic feature of OOP, if you want to call the base class constructor you have to do it explicitely from the derived class constructor.It works because all methods of the parent class are available at the derived class due to inheritance.

<?php

function Another() {
    
$this->y=5;
    
$this->Something();   //explicit call to base class constructor.
}

?>
A nice mechanism in OOP is to use Abstract Classes, abstract classes are classes that are not instanciable and has the only purpose to define an interface for its derived classes. Designers often use Abstract classes to force programmers to derive classes from certain base classes so they can be certain that the new classes have some desired functionality. There's no standard way to do that in PHP but:
If you do need this feature define the base class and put a "die" call in its constructor so you can be sure that the base class is not instanciable, now define the methods (interface) putting "die" statements in each one, so if in a derived class a programmer doesn't override the method then an error raises. Furthermore you might need to be sure since php has no types that some object is from a class derived from you base class, then add a method in the base class to identify the class (return "some id"), and verify this when you receive an object as an argument. Of course this doesn't work if the evil programmer oveerides the method in the derived class but genrally the problem is dealing with lazy programmers no evil ones! Of course is better to keep the base class unreachable from the programmers, just print the interface and make them work!
There're no destructors in PHP.
Overloading (which is different from overriding) is not supported in PHP. In OOP you "overload" a method when you define two/more methods with the same name but different number or type of parameters (depending upon the language). Php is a loosely typed language so overloading by types won't work, however overloading by number of parameters doesn't work either.
It's very nice sometimes in OOP to overload constructors so you can build the object in different ways (passing different number of arguments). A trick to do something like that in PHP is:

<?php

class Myclass {
    function 
Myclass() {
        
$name="Myclass".func_num_args();
        
$this->$name();
        
//Note that $this->$name() is usually wrong but here
        //$name is a string with the name of the method to call.
    
}

    function 
Myclass1($x) {
        
code;
    }
    function 
Myclass2($x,$y) {
        
code;
    }
}

?>
With this extra working in the class the use of the class is transparent to the user:
$obj1=new Myclass('1');      //Will call Myclass1
$obj2=new Myclass('1','2');  //Will call Myclass2
Sometimes this is very nice.

Polymorphism

Polymorphism is defined as the ability of an object to determine which method to invoke for an object passed as argument in runtime time. For example if you have a class figure which defines a method draw and derived classes circle and rectangle where you override the method draw you might have a function which expects an argument x and then call $x->draw(). If you have polymorphism the method draw called depends of the type of object you pass to the function. Polymorphism is very easy and natural in interpreted languages as PHP (try to imagine a C++ compiler generating code for this case, which method do you call? You don't know yet which type of object you have!, ok this is not the point). So PHP happily supports polymorphism.

<?php

function niceDrawing($x) {
    
//Supose this is a method of the class Board.
    
$x->draw();
}

$obj=new Circle(3,187);
$obj2=new Rectangle(4,5);

$board->niceDrawing($obj);    //will call the draw method of Circle.
$board->niceDrawing($obj2);   //will call the draw method of Rectangle.

?>

OOP Programming in PHP

Some "purists" will say PHP is not truly an object oriented language, which is true. PHP is a hybrid language where you can use OOP and traditional procedural programming. For large projects, however, you might want/need(?) to use "pure" OOP in PHP declaring classes, and using only objects and classes for your project. As larger and larger projects emerge the use of OOP may help, OOP code is easy to mantain, easy to understand and easy to reuse. Those are the foundations of software engineering. Applying those concepts to web based projects is the key to success in future web sites.

Advanced OOP Techniques in PHP

After reviewing the basic concepts of OOP I can show you some more advanced techniques:

Serializing

PHP doesn't support persistent objects, in OOP persistent objects are objects that keep its state and funcionality across multiple invocations of the application, this means having the ability of saving the object to a file or database and then loading the object back. The mechanism is known as serialization. PHP has a serialize method which can be called for objects, the serialize method returns a string representation of the object. However serialize saves the datamembers of the object but not the methods.
In PHP4 if you serialize the object to string $s, then destroy the object, and then unserialize the object to $obj you might still access the object methods! I don't recommend this because (a) The documentation doesn't guarrantee this beahaviour so in future versions it might not work. (b) This might lead to 'illusions' if you save the serialized version to disk and exit the script. In future runs of the script you can't unserialize the string to an object an expect the methods to be there because the string representation doesn't have the methods! Summarizing serializing in PHP is VERY useful to save the data members of an object just that. (You can serialize asociative arrays and arrays to save them to disk too).
Example:

<?php

$obj
=new Classfoo();
$str=serialize($obj);
// Save $str to disk

//...some months later

//Load str from disk
$obj2=unserialize($str)

?>
You have the datamembers recovered but not the methods (according to the documentation). This leads to $obj2->x as the only way to access the data members (you have no methods!) so don't try this at home.
There're some ways to fix the problem, I leave it up to you because they are too dirty for this neat article.
Full serialization is a feature I'd gladly welcome in PHP.

Using Classes to Manipulate Stored Data

One very nice thing of PHP and OOP is that you can easily define classes to manipulate certain things and the call the appropiate classes whenever you want. Suposse you have a html form where the user selects a product by selecting it's product ID, you have the data of the products in a database and you want to display the product, show its price, etc etc. You have products of different types, and the same action might have different meanings for different kind of products. For example showing a sound might mean playing it while for some other kind of products might mean to display a picture stored in the database. You might use OOP and PHP to code less and code better:
Define a class product, define which methods the class should have (example display), then define classes for each type of product which extends the product class (class SoundItem, class ViewableItem, etc...) override the methods you define in product in each of this classes make them do what you want. Name the classes according to the "type" column you store in the database for each product a typical product table might have (id,type,price,description,etc etc)... Then in the processing script you can retrieve the type from the database and instanciate an object of the class named type using:

<?php

$obj
=new $type();
$obj->action();

?>
This is a very nice feature of PHP, you might then call the display method of $obj or any other method regardless the type of object you have. With this technique you don't have to touch the processing script when you add a new type of object, just add a class to handle it. This is quite powerful, just define the methods all objects regardless of its type should have, implement them in different ways in different classes and use them for any type of object in the main script, no ifs, no 2 programmers in the same file, eternal happiness. Do you agree programming is easy, mainteinance is cheaper and reusability is real now?
If you command a group of programmers is easy to divide the tasks, each one might be responsable for a type of object and the class that handles it. Internacionalization can be done using this technique, apply the proper class according to a language field selected by the user, etc.

Copying and Cloning

When you create an object $obj you can copy the object by doing $obj2=$obj, the new object is a copy (not a reference) of $obj so it has the state $obj had in the moment the assignment was made. Sometimes you don't want this, you just want to create a new object of the same class as obj, calling the constructor of the new object as if you had used the new statement. This can be done in PHP using serialization and a base class that all other classes must extend.

Entering a Danger Zone

When you serialize an object you get a string which has a certain format, you may investigate it if you are curious, one of the things the string has is the name of the class (nice!) you may extract it using:

<?php

$herring
=serialize($obj);
$vec=explode(':',$herring);
$nam=str_replace("\"",'',$vec[2]);

?>
So suposse you create a class "Universe" and force that all classes must extend universe, you can define a method clone in Universe as:

<?php

class Universe {
    function clone() {
        
$herring=serialize($this);
        
$vec=explode(':',$herring);
        
$nam=str_replace("\"",'',$vec[2]);
        
$ret=new $nam;
        return 
$ret;
    }
}

//Then:

$obj=new Something();
    
//Something extends Universe !!
$other=$obj->clone();

?>
What you get is a new object of class Something created the same way as using new, the constructor is called, etc. I don't know if this is useful for you, but the Universe class which knows the name of the derived class is a nice concept to experiment. The only limit is your imagination.
Note: I'm using PHP4, something I write here may not work in PHP3.