Property VS reference: share a variable between class methods - php

Question

Let's say that I'm building a class (for a generic purpose) and I have two methods that work on common variables.
class renderElement
{
public function process()
{
$output = array();
$data = $this->supportMethod($output);
// do stuff with $output
}
public function supportMethod(&$processed_output)
{
// do stuff with $processed_output
}
}
I want to use the same variable into both methods; let's say $output.
I'm curious to know the theory behind the choice of eventually rely on a class property (like following) or when instead is ok (or better) to pass the variable as reference.
class renderElement
{
private $output = array();
public function process()
{
$data = $this->supportMethod();
// do stuff with $this->output
}
public function supportMethod()
{
// do stuff with $this->output
}
}
Advantages? Disadvantages? Design suggestions?
Suggested lectures about this kind of design choices?
[ EDIT ]
I would add that the variable/property should obviously have sense as an object property. I'm not talking about variables for just supporting data processing.

Answer 1

An object fundamentally consists of two things, behaviour and state.
Methods of your class define the behaviour of your object.
Attributes of your class define the state of your object.
In your first example, $output isn't a part of the object's state, so it will not persist throughout the life of the object.
In your second example, $output will persist throughout the life of the object since it is a part of the object's state.
Take this example...
class Shape
{
protected $sides;
public function getNumberOfSides()
{
return $this->sides;
}
}
class Triangle extends Shape
{
public function __construct()
{
$this->sides = 3;
}
}
class Square extends Shape
{
public function __construct()
{
$this->sides = 4;
}
}
$shape = new Square;
echo $shape->getNumberOfSides(); // Returns 4
$shape = new Triangle;
echo $shape->getNumberOfSides(); // Returns 3
$sides (number of sides) is an intrinsic part of a shapes nature, therefore it is appropriate for it to be a part of the object's state throughout the life of the object. This doesn't really have anything to do with the convenience of passing parameters between methods in the same class.