Immutable objects in PHP?

An immutable object cannot be changed after its initial creation so having setter methods makes no sense as it goes against that base principle.

You could implement some workarounds to simulate immutability in PHP by manipulating class member visibility and overriding the magic __set() method but its not guaranteed immutable as immutability is not a feature of the PHP language.

I believe someone once wrote an extension to provide an immutable value type in PHP though so you could google for that.

In my opinion objects should be immutable for value objects. Other than that it does not have much benefits unless you're sharing your object across your whole application.

There is some wrong answers here, an immutable object can have setters. Here's some implementation of immutable objects in PHP.

Example #1.

class ImmutableValueObject
{
    private $val1;
    private $val2;

    public function __construct($val1, $val2)
    {
        $this->val1 = $val1;
        $this->val2 = $val2;
    }

    public function getVal1()
    {
        return $this->val1;
    }

    public function getVal2()
    {
        return $this->val2;
    }
}

As you can see once instantiated you cannot changed any value.

Example 2: with setters:

class ImmutableValueObject
{
    private $val1;
    private $val2;

    public function __construct($val1, $val2)
    {
        $this->val1 = $val1;
        $this->val2 = $val2;
    }

    public function getVal1()
    {
        return $this->val1;
    }

    public function withVal1($val1)
    {
        $copy = clone $this;
        $copy->val1 = $val1;

        return $copy;    // here's the trick: you return a new instance!
    }

    public function getVal2()
    {
        return $this->val2;
    }

    public function withVal2($val2)
    {
        $copy = clone $this;
        $copy->val2 = $val2;

        return $copy;
    }
}

There is several implementation possible and this is by no means an exclusive list. And remember that with Reflection there is always a way to get around that in PHP, so immutability is all in your head in the end!

It is also often good practice to put immutable objects as final.

EDIT:

• changed setX for withX
• added comment about final

Immutable objects don't have setter methods. Period.

Everyone will expect a setXyz() method to have a void return type (or return nothing in loosely typed languages). If you do add setter methods to your immutable object it will confuse the hell out of people and lead to ugly bugs.