PHP OOP constructing class property form method in same class

Question

For some reason I can't get this to work:

<?php 
class Number{
    public $number;
    public $number_added;


    public function __construct(){
        $this->number_added = $this->add_two();
    }

    public function add_two(){
        return $this->number + 2;
    }
}   
?>

$this->number is set from Database, $this->number_two should be DB value + 2. However, when I echo $this->number_added, it returns two. The $number value was initialized correctly. This is a simplified example of my problem just to see if what I am trying to do possible? PHP OOP beginner.

Answer 1

You aren't setting the $number property anywhere prior to its use in add_two() (via the constructor), therefore PHP evaluates it as 0 during the addition.

You should pass in initial state during object construction, for example

public function __construct($number) {
    $this->number = $number;

    $this->number_added = $this->add_two();
}

Update

Allow me to illustrate the problem. Here's your current code and how I imagine you're using it

$number = 2;

$obj = new Number();
// right here, $obj->number is null (0 in a numeric sense)
// as your constructor calls add_two(), $obj->number_added is 2 (0 + 2)

$obj->number = $number;
// now $obj->number is 2 whilst $obj->number_added remains 2

Using my updated constructor, here is what happens

$number = 2;

$obj = new Number($number);
// $obj->number is set to $number (2) and a call to add_two() is made
// therefore $obj->number_added is 4