Object Oriented Programming in PHP
This article introduces Object Oriented Programming (OOP) in PHP. Luis shows you how to code less and better by using some OOP concepts and PHP tricks.Object Oriented Programming in any language is the use of objects to represent functional parts of an application and real life entities. For example you may have a Person object to hold the data related to a person and even provide some functionality that this person may be capable of.
Object Oriented Programming has long been used in games to represent the objects such as a User or an Enemy, or even a Weapon. This amazing way of programming has proven just as useful in software and web development.
In my opinion any OOP language should have:
Abstract data types and information hiding
Inheritance
Polymorphism
This can all be done using PHP 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 separate
// 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 standardized one is useful.
Object Oriented Programming in PHP - Data Members and Functions
Data members are defined in PHP using a "var" declaration inside the class and they 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 like:
$obj->setX(5);
$see=$obj->getX();
The setX member function assigns 5 to the x data member in the object obj (not in the class), then getX returns its value; 5 in this case.
You can access the data members from the object reference using for example: $obj->x=6. However, this is not a very good OOP practice.
I encourage you to set data members by defining methods to set them and access the data members by using retrieving methods.
You'll be a good OOP programmer if you consider data members inaccessible 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
Inheritance is easy in PHP using the extends keyword:
<?php
class Another extends Something {
var $y;
function setY($v) {
// Methods start in lowercase then use uppercase initials to
// separate words in the method name example getValueOfArea()
this->y=$v;
}
function getY() {
return $this->y;
}
}
?>
Objects of the class "Another" now have 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 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.
Object Oriented Programming in PHP - Constructors
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 using:
$obj=new Something(6);
...and the constructor automatically assigns 6 to the data member x.
Constructors and methods are normal PHP functions so you can use default arguments.
function Something($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 assigned from 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 explicitly 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.
}
?>
Abstract Classes
A nice mechanism in OOP is the use of Abstract Classes; abstract classes are classes that cannot be instantiated and the only purpose is 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, just define the base class and put a "die" call in its constructor so you can be sure that the base class is never instantiated.
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 is raised.
Furthermore you might need to be sure, since PHP has no types, that some object is from a class derived from your 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 overrides the method in the derived class but generally the problem is dealing with lazy programmers not evil ones! Of course it’s better to keep the base class unreachable from the programmers, just print the interface and make them work!
There're no destructors in PHP.
Object Oriented Programming in PHP - Overloading
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.
?>
Object Oriented Programming in PHP - 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 maintain, 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:
Serialization
PHP doesn't support persistent objects; in OOP persistent objects are objects that keep its state and functionality across multiple invocations of the application. This means having the ability to save the object to a file or database and then load 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 data members of the object but not the methods.
In PHP4 if you serialize the object to string $s, then destroy the object, then unserialize the object to $obj you might still access the object methods! I don't recommend this because:
The documentation doesn't guarantee this behaviour so in future versions it might not work.
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 and expect the methods to be there because the string representation doesn't have the methods.
You can serialize associative 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 data members 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 but 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 about PHP and OOP is that you can easily define classes to manipulate certain things and the call the appropriate classes whenever you want.
Suppose you have a HTML form where the user selects a product by selecting its product ID, you have the data of the products in a database and you want to display the product, show its price, 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.).
Then in the processing script you can retrieve the type from the database and instantiate 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, maintenance is cheaper and reusability is real now?
If you command a group of programmers it's easy to divide the tasks, each one might be responsible for a type of object and the class that handles it. Internationalization can be done using this technique, apply the proper class according to a language field selected by the user, etc.
Object Oriented Programming in PHP - 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 this if you are curious. One of the things the string has is the name of the class (nice!), you can extract it using:
<?php
$herring=serialize($obj);
$vec=explode(':',$herring);
$nam=str_replace(""",'',$vec[2]);
?>
So suppose 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 with. The only limit is your imagination.
Note: I'm using PHP4, some of these examples may not work in PHP3.