Wojciech Sznapka has a quick post to his site today looking at a possible implementation of Value Objects (immutable objects) in PHP applications.

Value objects are one of building blocks in Domain Driven Design. They represents a value and does not have an identity. That said, two value objects are equal if their values are equal. Other important feature is that Value Objects are immutable, i.e. they can not be modified after creation. [...] This post isn't about obvious advantages of representing domain logic with support of Value Object. As well, we wouldn't elaborate here about pros and cons of immutable objects. I'd rather would like to show an attempt to change Value Object.

His change method isn't so much a "change" as a "duplicate with new values" process. In his example he creates a EmailValueObject with "host" and "mailbox" properties. This object has a "changemailbox" method that seems to update the "mailbox" property, but in actuality clones the current object with a new "mailbox" value in the constructor.