Object Oriented Programming in PHP -3 [The $this keyword]

What is $this Keyword

In PHP, $this keyword references the current object of the class. The $this keyword allows you to access the properties and methods of the current object within the class using the object operator (->):

$this->property

$this->method()

How $this Keyword Works

During code execution when an object calls the method ‘setdata’. The keyword ‘this’ is replaced by the object handler “obj.” (See the image below).

Scope of $this Keyword

The $this keyword is only available within a class. It doesn’t exist outside of the class. If you attempt to use the $this outside of a class, you’ll get an error.

Rules to use $this Keyword

When you access an object property using the $this keyword, you use the $ with the this keyword only. And you don’t use the $ with the property name. For example:

$this->balance

The following shows the BankAccount class:

<?php

class BankAccount
{
	public $accountNumber;

	public $balance;

	public function deposit($amount)
	{
		if ($amount > 0) {
			$this->balance += $amount;
		}
	}



	public function withdraw($amount)
	{
		if ($amount <= $this->balance) {
			$this->balance -= $amount;
			return true;
		}
                return false;

	}

}

In the above example, we access the balance property via the $this keyword inside the deposit() and withdraw() methods.

In the first tutorial of this series, we learned how to group the code related to a certain topic into one single class. As an example, we wrote a Car class that groups the code that handles cars.

class Car {

  public $comp;
  public $color = 'beige';
  public $hasSunRoof = true;
 
  public function hello() 
  {
    return "beep";
  }
}

We also created two objects out of the class in order to be able to use its code by those two objects.

$bmw = new Car ();
$mercedes = new Car ();

The $this keyword

The $this keyword indicates that we use the class’s own methods and properties, and allows us to have access to them within the class’s scope.

The $this keyword allows us to approach the class properties and methods from within the class using the following syntax:

$this -> propertyName;
$this -> methodName();

Let’s illustrate what we have just said on the Car class. We will enable the hello() method to approach the class’s own properties by using the $this keyword.

$this -> comp

$this -> color

That’s what the code looks like:

class Car {

    // The properties
    public $comp;
    public $color = 'beige';
    public $hasSunRoof = true;
 
    // The method can now approach the class properties
    // with the $this keyword
    public function hello()
    {
      return "Beep I am a <i>" . $this -> comp . "</i>, and I am <i>" .
        $this -> color;
    }
}

$bmw = new Car();
$mercedes = new Car ();

$bmw -> comp = "BMW";
$bmw -> color = "blue";
 
$mercedes -> comp = "Mercedes Benz";
$mercedes -> color = "green";

echo $bmw -> hello();

Result:

echo $mercedes -> hello();

Result:

Beep I am a Mercedes Benz, and I am green.

This is the code that we have written in this tutorial:

class Car {

    // The properties
    public $comp;
    public $color = 'beige';
    public $hasSunRoof = true;
 
    // The method that says hello
    public function hello()
    {
      return "Beep I am a <i>" . $this -> comp . 
         "</i>, and I am <i>" . $this -> color;
    }
}
 
// We can now create an object from the class.
$bmw = new Car();
$mercedes = new Car();
 
// Set the values of the class properties.
$bmw -> color = 'blue';
$bmw -> comp = "BMW";
$mercedes -> comp = "Mercedes Benz";
 
// Call the hello method for the $bmw object.
echo $bmw -> hello();