Question

I’m confused about the following output

class A{
    public $v = 10;    
    function add($number){
        $this->v +=$number;
    }

}
$a = new A;
echo $a->v . "\n";
$a->add(5);
echo $a->v . "\n";

Why does the second line output 15 instead of 10? I thought changes made to values inside a function do not propagate outside of the function unless you pass by reference.

Answer 1

Your remark “changes made to values inside a function do not propagate outside of the function” goes for parameters. Which can be passed ‘by value’ or ‘by reference’ for instance:

class A{
    public $v = 10;    
    function add($number){
        $number += $this->v;
    }

$b = 5;
$a->add($b);
echo $b; // Will still be 5;

In the sample above, $b will become 15 only if it is passed by reference to the function.

In your case, you’re not modifying the parameter at all. You’re not modifying a local variable either.

You’re modifying the property v of the object. $this is a special variable, that is local to the function, but references the object. The actual variable you modify is not $this, nor the parameter, but a property of $a.