PHP and Object Oriented Programming

An object is a self-contained piece of functionality that can be easily used, and re-used as the building blocks for a software application.

Objects consist of data variables and functions (called methods) that can be accessed and called on the object to perform tasks. These are collectively referred to as members.

What is a Class?

Much as a blueprint or architect’s drawing defines what an item or a building will look like once it has been constructed, a class defines what an object will look like when it is created. It defines, for example, what the methods will do and what the member variables will be.

How is an Object Created from a Class?

The process of creating an object from the class ‘blueprint’ is called instantiation. Essentially, you instantiate an instance of the class and give that instance a name by which you will refer to it when accessing members and calling methods. You can create as many object instances of a class as you desire. Objects are instantiated using the new keyword. For example, to create an instance of a class called bankAccount we would write:

$accountObject = new bankAccount();

In the above example we now have an object called $accountObjhect of type bankAccount.

What is sub-classing?

It is possible to build classes that are derived from other classes, extending the functionality of the parent class to make it specific to a particular requirement. For example you might have a vehicle class which contains the attributes common to all vehicles, and a subclass called car which inherits all the generic vehicle attributes but adds some its own car specific methods and properties.

PHP Class Constructors and Destructors

The constructor and destructor are really just functions that get called when the object is created and destroyed and are defined in the class body using the function keyword. This needs to be prefixed with the public qualifier. This means the method is accessible to code outside of the object. The default names for the constructor and destructor are __construct and __destruct respectively. Both methods can take arguments to be used in initializing the object. These are declared in the same way arguments are defined in any function (see PHP Functions for details).

Creating Members in a PHP Class

Class members are essentially variables and methods embedded into the class. Members can be public or private and static or variable.

public members can be accessed from outside the object. private members can only be accessed by methods contained in the class. This is the key to what is called data encapsulation. Object-oriented programming convention dictates that data should be encapsulated in the class and accessed and set only through the methods of the class (typically called getters and setters).

Members declared as static are immutable, in that once defined they cannot be changed (much like constants). Members and functions are prefixed with public, private and static when declared in the class. The default is public non-static.

Subclassing in PHP

Once a class has been defined it is possible to create a new class derived from it that extends the functionality of the original class. The parent class is called the superclass and the child the subclass. The whole process is referred to as subclassing.

A subclass of a class can be defined using the extends keyword when declaring the subclass.

PHP Object Serialization

One of the interesting features of object oriented programming is the ability to take a snapshot of the current state of an object and then save that object to a file, or even transmit it over a network to another process where it will be re-activated. This concept is known in the object oriented world as object serialization.

All objects have built-in method called __sleep that is called before serialization. If you need your object to perform any housekeeping before being serialized you will need to override this method.

An object is serialized using the serialize() function and unserialized, not surprisingly, using the unserialize() function.

Getting Information about a PHP Object

There are number of ways to find out information about classes.

An array of classes to which your script has access can be obtained using the get_declared_classes() function. The class_exists() function can be passed the name of a class to find out if such a class exists. A list of methods in a class can be obtained by passing the class name through to the get_class_methods() function.

It is also possible to find out if a class has a parent class using the get_parent_class() function which will either return the name of the parent class, or an empty string if no parent exists.

The method_exists() function, when passed an object pointer and method name as arguments, will return a true or false value indicating the existence of the method.