What is an example of a Value Object in PHP?

Question

I have read plenty of C# examples of Value Objects, and I understand that it is an "object" that is identified by it's values. If a value changes, the object is "new".

However, that doesn't seem to make sense when it comes to PHP...either that, or I'm just not making the connection.

Is a Value Object just a string?

Psilocybin

Not everything in PHP is an object. – Colorblind

I realize that. I'm just trying to make sense of some tutorials I've been reading that speak of Value Objects. – Psilocybin

But not everything in PHP is an object, which results in the comparison being invalid. – Colorblind

If I am reading "language agnostic" tutorials, what should I replace the phrase "Value Object" with in my mind? When someone says a Person object has an Address Value object - should I assume that in my world that simply translates to a property inside the Person? Thanks :) – Psilocybin

@Ignacio Vazquez-Abrams: not everything in C# is an object either, and that has absolutely nothing to do with this question. – Daudet

Answer 1

To put this into context, in many OO languages, objects are compared by their identity. In pseudocode:

bar = new Foo
baz = new Foo

bar == baz  // false

Even though both objects are basically the same if you just look at their values, they're not considered to be identical, because they are separate instances. To demonstrate:

bar = new Foo
baz = bar

bar == baz  // true

Now:

In computer science, a value object is a small simple object, like money or a date range, whose equality isn't based on identity.

http://en.wikipedia.org/wiki/Value_object

This would be a demonstration of "value objects":

address1 = new Address('Main street 42')
address2 = new Address('Main street 42')

address1 == address2  // true

Because the values are the same, both objects are considered equal, even if they're separate instances.

PHP does not have a separate concept of "value objects", it only has one type of object. Its comparison operator can make that distinction though:

When using the comparison operator (==), object variables are compared in a simple manner, namely: Two object instances are equal if they have the same attributes and values, and are instances of the same class.

http://www.php.net/manual/en/language.oop5.object-comparison.php

E.g.:

$address1 = new Address('Main street 42');
$address2 = new Address('Main street 42');

$address1 == $address2;  // true     equal...
$address1 === $address2;  // false   ...but not identical

Southpaw

@johnnietheblack: Note that in C# the distinction also rests mainly on the comparison method Equals(), which on System.Object compares for identity, but which System.ValueType overrides to compare content. – Daudet

There are a lot of repositories that makes VO in PHP. This is my implementation: github.com/sensorario/value-object – Execution

Answer 2

PHP used to have value objects but that was back in PHP4. See http://3v4l.org/ghI8G

Ankylostomiasis