WE USE “THIS” ON OBJECTS

legs = $l;
    $this->wings = $w;
  }

  function introduce () {
    // $this refers to the OBJECT'S properties
    echo "This animal has $this->legs legs and $this->wings wings.
";
  }
}

$doge = new Animal(4, 0);
$chicken = new Animal(2, 2);
$doge->introduce();
$chicken->introduce();
?>

This animal has 4 legs and 0 wings.
This animal has 2 legs and 2 wings.

Let us start with the easier $this first. Remember that classes are kind of like templates in OOP? Where we define the common properties and functions? Objects that are created from the class template will also adopt these properties and functions, but they can hold different values – This is where we use $this to refer to the object’s functions and properties.

WE USE “SELF” ON CLASSES

THIS WILL NOT WORK ON CLASSES

So when do we use the self keyword? As you have read earlier in the introduction, when we want to refer to the claass instead. So let us start with a negative example of what happens when we use $this on the class itself:

legs = $l;
    $this->wings = $w;
  }

  function introduce () {
    echo "This animal has $this->legs legs and $this->wings wings.
";
  }
}

// Directly calling the introduce function of class Animal
// ERROR - $this refers to properties of the object, NOT the class itself
Animal::introduce();
?>

Fatal error: Uncaught Error: Using $this when not in object context in D:\http\test\2a-bad-self.php:12 
Stack trace: #0 D:\http\test\2a-bad-self.php(17): Animal::introduce() #1 {main} thrown in D:\http\test\2a-bad-self.php on line 12

SELF WILL NOT WORK ON NON-STATIC

As expected, PHP will complain “using $this when not in object context” – We should be using self instead:

legs = $l; 
    $this->wings = $w;
  }
 
  function introduce () {
    echo "This animal has " . self::$legs . " legs and " . self::$wings . " wings.
";
  }
}

// ERROR - property is not static
Animal::introduce();
?>

Fatal error: Uncaught Error: Access to undeclared static property: Animal::$legs in D:\http\test\2b-bad-self.php:12
Stack trace: #0 D:\http\test\2b-bad-self.php(17): Animal::introduce() #1 {main} thrown in D:\http\test\2b-bad-self.php on line 12

AND oopsies again, self can only access static members.

THE PROPER SELF

So finally, here is a proper example of self:

legs = $l; 
    $this->wings = $w;
  }

  function introduce () {
    echo "This animal has " . self::$legs . " legs and " . self::$wings . " wings.
";
  }
}

// OK - 0 legs and 0 wings
Animal::introduce();

// NOTE - static class properties cannot be changed directly by objects
// Take extra care in what you "static"
$doge = new Animal(999, 888);
// 0 legs and 0 wings
$doge->introduce();
?>

THIS VS SELF

EXTRA – PARENT

As a small added extra, here is something that is good for you to know as well – Apart from self, we can use the parent keyword to refer to the ancestor:

legs = $l;
    $this->wings = $w;
  }

  function introduce () {
    echo "This animal has " . self::$heart . " heart, " .$this->legs. " legs, and " . $this->wings . " wings.
";
  }
}

class Doge extends Animal {
  // This will override the parent introduce function
  function introduce () {
    echo "BORK!
";
    // But we can still call it with the parent keyword
    parent::introduce();
  }
}

$pupper = new Doge(3, 5);
$pupper->introduce();
?>

REFERENCES

• Classes and objects in PHP
• Scope Resolution Operator in PHP
• Static keyword in PHP

THE INFOGRAPHIC

SOURCE CODE DOWNLOAD

Click here to download all the example source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.
 

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