I'm wondering what do we call this kind of assignment.

<?php
class SimpleClass
{
    public $var1;
    public $var2;
    public $var3;

    public function SimpleClass()
    {
        $this->var1 = 'one';
        $this->var2 = 'two';
        $this->var3 = 'three';
    }
}

function test()
{
    $objSc = new SimpleClass();
    $objSc->var4 = 'WTF?!'; # <-- what do we call this?
    var_dump($objSc);
}

test();
?>

Better with references or links. Thanks in advance!