Objects

PHP is an object oriented language, although it does not have to be used as one, since most PHP functions are not object oriented.

In object oriented programming, a class is a definition of an object, whereas an object is an instance of an object, meaning that from one class you can create many objects.

For example, let's define a class of a student.

class Student {
    // constructor
    public function __construct($first_name, $last_name) {
        $this->first_name = $first_name;
        $this->last_name = $last_name;
    }

    public function say_name() {
        echo "My name is " . $this->first_name . " " . $this->last_name . ".\n";
    }
}

$alex = new Student("Alex", "Jones");
$alex->say_name();

Let's analyze the code. Notice that the Student class has a constructor function, which is executed when the object is created. The constructor receives arguments which are later provided when constructing the object with the new keyword.

After we have constructed the object into the variable $alex we can now use the object's methods.

We implemented an object method say_name, which prints out the name of the student. Notice that the say_name function does not receive any arguments, but it does have access to the first and last name of the student, because they were previously defined in the constructor.

Here are some important definitions related to objects:

• Classes define how objects behave. Classes do not contain data.
• Objects are instances of classes, which contain data.
• Members are variables that belong to an object.
• Methods are functions that belong to an object, and have access to its members.
• Constructor is a special method that is executed when an object is created.

Inheritance

The most important feature of object oriented programming is inheritance. This feature allows us to reuse code we've written and extend it. For example, let's say we want to be able to define a math student, which also knows how to sum two numbers.

class Student {
    // constructor
    public function __construct($first_name, $last_name) {
        $this->first_name = $first_name;
        $this->last_name = $last_name;
    }

    public function say_name() {
        echo "My name is " . $this->first_name . " " . $this->last_name . ".\n";
    }
}

$alex = new Student("Alex", "Jones");
$alex->say_name();

class MathStudent extends Student {
    function sum_numbers($first_number, $second_number) {
        $sum = $first_number + $second_number;
        echo $this->first_name . " says that " . $first_number . " + " . $second_number . " is " . $sum;
    }
}

$eric = new MathStudent("Eric", "Chang");
$eric->say_name();
$eric->sum_numbers(3, 5);

Notice that Eric's object also has the same constructor and the say_name function, in addition to a new method called sum_numbers, which causes Eric to calculate the sum of two numbers. Also notice that the new function has access to the same members we have previously defined in the Student class (first_name, last_name).

Public and private functions

We can use the public and private modifiers respectively to define functions which can be accessed from outside the object or not, for encapsulation purposes. This allows to better define how objects should be used, to separate between functions which are used for internal use, as opposed to an external interface.

class Student {
    // constructor should be public
    public function __construct($first_name, $last_name) {
        $this->first_name = $first_name;
        $this->last_name = $last_name;
    }

    // for external use
    public function say_name() {
        echo "My name is " . $this->full_name() . "\n";
    }

    // for internal use
    private function full_name() {
        return $this->first_name . " " . $this->last_name;
    }
}

$alex = new Student("Alex", "Jones");

$alex->say_name();

// this will not work
// echo $alex->full_name();

Exercise

Create a class called Car with a constructor that receives the brand and make year of the car, and a function called print_details that prints out the details of the car.

For example, for a 2006 Toyota car, the following line would be printed out:

This car is a 2006 Toyota.