Question

Please tell me why:

class ClassOne {
    protected $a = 10;

    public function changeValue($b) {
        $this->a = $b;
    }
}

class ClassTwo extends ClassOne {
    protected $b = 10;

    public function changeValue($b) {
        $this->b = 10;
        parent::changeValue($this->a + $this->b);
    }

    public function displayValues() {
        print "a: {$this->a}, b: {$this->b}\n";
    }
}

$obj = new ClassTwo();

$obj->changeValue(20);
$obj->changeValue(10);    
$obj->displayValues();

Prints a: 30 and b: 10.

I would appreciate an elaborate response. Thank you 🙂

On a side note: This is actually an exam question I’ve looked over and did not quite understand it. Thank you for your responses.

Answer 1

I don’t know how to summarize this execution besides “it’s executing the logic as you’ve written”.

I’ve added comments to each call to show the intermediary steps:

$obj = new ClassTwo(); // a = 10, b = 10

$obj->changeValue(20); // Sets b = 10, a = 20 (a = 10 + 10 = 20)
$obj->changeValue(10); // Sets b = 10, a = 30 (a = 20 + 10 = 30)
                                                   ^
                                                   Previous value of a
$obj->displayValues();

You may be confused by this:

public function changeValue($b) {
    $this->b = 10;
    parent::changeValue($this->a + $this->b);
}

Because $b is never assigned to anything. So it doesn’t matter what you pass to changeValue, you’ll always get 30, 10 when you call changeValue twice.

E.g. :

$obj->changeValue(0); 
$obj->changeValue(0);

Will still output a: 30 and b: 10