Nowadays that very procedural approach is less common in PHP, so this article takes a look at some of the basic object oriented features available in the language and shows some examples of using them with code examples.
Using OOP (Object Orientated Programming) enables us to architect our systems much more clearly, and to make them more manageable and more maintainable. This technique also allows us to separate form from function to create clean, navigable codebases with plenty of opportunities to reuse code, apply design patterns and bring in concepts from other brances of computer science.
Objects vs Classes
While the terms "object" and "class" are often used almost interchangeably in the world of software, there is a definite conceptual difference between the two. A class is the blueprint or recipe; it describes what the object should be, have and do. So a class might look like this:class Elephpant {
public $colour;
public function dance() {
echo "elephpant dances!\n";
return true;
}
}An object, on the other hand, is an actual instantiation of the class – its an actual thing, with values and behaviours. In the example below, $ele is the object:
include('elephpant.php');
$ele = new Elephpant();include('elephpant.php');
$ele = new Elephpant();
print_r($ele);
Elephpant Object
(
[colour] =>
)Using Objects, Their Properties and Methods
Our class, Elephpant, has a property and a method (OO-speak for "function") already defined inside it, so how can we interact with these? Let's start by setting the colour property; we use the object operator which is a hyphen followed by a greater than sign.include('elephpant.php');
$ele = new Elephpant();
// set the colour property
$ele->colour = "blue";
// now use that property
echo "The elephpant is " . $ele->colour;Similarly we can call the method using the same operator – the brackets after the call let PHP know its a method rather than a property, and if we had parameters to pass in they'd go between the brackets. Something like this:
include('elephpant.php');
$ele = new Elephpant();
// call the dance method
$ele->dance();Inheritance
Now we know how to create and interact with objects, let's step things up a bit and look at how we can create objects which are similar in some ways and different in others, using inheritance. If you are accustomed to OOP from any other programming languages then this will seem very familiar to you, so here is a quick look at how this can be done in PHP. We already have our Elephpant class declared, so let's add a Penguin class as well. They will both inherit from the parent class Animal, which looks like this:class Animal{
public $type = "animal";
public function dance() {
echo $this->type . " dances!\n";
return true;
}
}The animal has a "type" property, and a dance() method. It uses the type property in the dance() method to create the output, using $this to refer to the current object. In PHP, $this is a special keyword which refers to the current object from within its own class.
Now we can create a Penguin class that inherits from this general Animal class, by using the extends keyword to denote its parent:
class Penguin extends Animal {
public $type = "penguin";
}The penguin class would inherit the type property set to "animal" if we didn't override it by setting the property again in this class. However even without declaring a dance() method, the penguin can dance!
include('animal.php');
include('penguin.php');
$tux = new Penguin();
// make tux dance
$tux->dance();Access Modifiers
Take a look at the previous example. The type is set in the class, but we could easily set it from our main code if we wanted to. Imagine if we put $tux->type = "giraffe" before we asked him to dance! Sometimes this is the desired behaviour, and sometimes it isn't. To allow us to control whether properties can be changed outside a class, PHP gives us access modifiers. An example of these are the "public" keywords you see in the classes shown above. To prevent the type property being edited from outside the class, we can declare the Penguin type to be private, so the class now looks like this:class Penguin extends Animal {
private $type = "penguin";
}The following code listing shows us trying to set the now-private type property, and is followed by the resulting output.
include('animal.php');
include('private_penguin.php');
$tux = new Penguin();
// change the type
$tux->type = "linux penguin";
// make tux dance
$tux->dance();
Fatal error: Access level to Penguin::$type must be public (as in class Animal) in /home/lorna/.../OOP/private_penguin.php on line 5Access modifiers can be applied to properties and to methods and to properties. The options are public, private and protected. In PHP4, these weren't available and everything is public. As a result, and to maintain backwards compatibility, if an access modifier isn't specified then the property or method defaults to being public. This isn't recommended practice however, and it is best to be explicit about which is intended.
Public: The public access modifier means that properties and methods can be accessed from anywhere, within the scope of the object itself, and also from outside code operating on an object.
Private: The method or property is only available from within the scope of this specific class. Before using this option, read on to find out about the "protected" access modifier.
Protected: The method or property is available from within this class, and from within any classes with extend or implement this class. This is ideal where you don't want external code to change the class, but you do want to be able to extend the class later and take advantage of this property or method. Protected is more flexible than private and almost always the better choice.
Using these access modifiers we can control where our class methods and properties can be accessed from. We looked at an example of properties and this works in the same way for method calls – they cannot be accessed from outside of the class definition unless they are declared to be public. It can be very useful indeed to declare methods as protected, where they are internal helper methods used by other class methods but not intended to be accessed directly. In PHP4 there was no support for this and so the internal methods were sometimes named with an underscore to hint that this was their intended use – it is still possible to see this naming convention in use today, although it is not needed.
