In this tutorial you will learn about class attributes and how to declare & use them in PHP5 classes.

Definition of an class attribute
An attribute is also know as data members and is used to hold data of a class. The data that it holds are specific to the nature of the class in which it has been defined. For example, a Customer class would hold data related to a customer, an Order class would hold data related a an order.

Defining an attribute is as easy as declaring a variable within the body of the class. At the time of declaring a data member/variable within the body of the class, it is also possible to assign a default value to the attribute or data member.

Attributes can either be public, private or protected - the default being public. These are called Access Specifiers. You will learn more about access specifiers in PHP5 OOPS tutorials ahead.

Example Code:

class Customer {
	private $first_name, $last_name;
	private $outstanding_amount = 0; //example of default value
 
	public function getData($first_name, $last_name) {
		$this->first_name = $first_name;
		$this->last_name = $last_name;
	}
 
	public function printData() {
		echo $this->first_name . " : " . $this->last_name;
	}
}
 
$c1 = new Customer();

In the above example $first_name, $last_name and $outstanding_amount are data members or attributes of the class Customer. Of these attributes, $outstanding_amount has a default value of zero assigned to it. This means that when an object of the Customer class has been created, $first_name and $last_name will be blank whereas; $outstanding_amount will be initialized with default value of zero.

Let’s take a problem statement and define a PHP5 Class for it.

Problem Statement:
Define a class to encapsulate Student. The data required to be captured is Student’s first name, last name, date of birth, address and telephone number.

Solution:

class Student {
	public $first_name;
	public $last_name;
	public $date_of_birth;
	public $address;
	public $telephone;
}

In the above example $first_name, $last_name, $date_of_birth, $address and $telephone are all data members of class Student. These data members define the data that one Student Object will store. Therefore, you can create two objects of a Student class having different data as follows:

//creating an object $s1 of Student class and assigning data
$s1 = new Student();
$s1->first_name = "Sunil";
$s1->last_name = "Bhatia";
$s1->date_of_birth = "01/01/1900";
$s1->address = "India";
$s1->telephone = "9999999999";
 
//creating an object $s2 of Student class and assigning data
$s2 = new Student();
$s2->first_name = "Vishal";
$s2->last_name = "Thakur";
$s2->date_of_birth = "01/01/1910";
$s2->address = "USA";
$s2->telephone = "8888888888";

In the above example, we create two objects of the Student class $s1 and $s2. Both these objects have their own memory space within which they store their respective data. Because each data member is ‘public‘ you can directly access the data members of the Student Class using the arrow operator (reference operator) i.e. $s1->first_name = “Sunil”; You will learn more about public, private and protected in the PHP5 tutorials ahead on Access Specifiers.

The diagram below explains the above example:

Objects as Attributes

In addition to declaring attributes as intrinsic data types (int, string, etc), you can also declare data members as objects of another class. This is called aggregation in Object Oriented Analysis and Design (OOAD). Lets look at an example below:

class Customer {
	private $first_name, $last_name;
	private $outstanding_amount = 0; //example of default value
 
	public function getData($first_name, $last_name) {
		$this->first_name = $first_name;
		$this->last_name = $last_name;
	}
 
	public function printData() {
		echo $this->first_name . " : " . $this->last_name;
	}
}
 
class Order {
	private $order_id;
	private $customer;
 
	public setCust(Customer $c) {
		$this->customer = $c;
	}
}
 
$c1 = new Customer();
$o1 = new Order();
 
$o1->setCust($c1); //storing $c1 object in $o1 order object

In the above example setCust() method accepts a Customer type of parameter which is stored internally in the $customer data member.

The advantage of the above method is that it allows you to change the customer object independently of the order object. Imagine having to add 3 - 4 new data members to the Customer object. You only have to modify the Customer object without having the need to modify the Order Object.

In the next tutorial you will learn about PHP5 Class Method.

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