Object Cloning

Creating a copy of an object with fully replicated properties is not always the wanted behavior. A good example of the need for copy constructors is if you have an object which represents a GTK window and the object holds the resource of this GTK window, when you create a duplicate you might want to create a new window with the same properties and have the new object hold the resource of the new window. Another example is if your object holds a reference to another object which it uses and when you replicate the parent object you want to create a new instance of this other object so that the replica has its own separate copy.

An object copy is created by using the clone keyword (which calls the object's __clone() method if possible).

$copy_of_object = clone $object;

When an object is cloned, PHP will perform a shallow copy of all of the object's properties. Any properties that are references to other variables will remain references.

Once the cloning is complete, if a __clone() method is defined, then the newly created object's __clone() method will be called, to allow any necessary properties to be changed.

Cloning an object

<?php
class SubObject
{
    static $instances = 0;
    public $instance;

    public function __construct() {
        $this->instance = ++self::$instances;
    }

    public function __clone() {
        $this->instance = ++self::$instances;
    }
}

class MyCloneable
{
    public $object1;
    public $object2;

    function __clone()
    {
        // Force a copy of this->object, otherwise
        // it will point to same object.
        $this->object1 = clone $this->object1;
    }
}

$obj = new MyCloneable();

$obj->object1 = new SubObject();
$obj->object2 = new SubObject();

$obj2 = clone $obj;


print "Original Object:\n";
print_r($obj);

print "Cloned Object:\n";
print_r($obj2);

?> 

The above example will output:

Original Object:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 1
        )

    [object2] => SubObject Object
        (
            [instance] => 2
        )

)
Cloned Object:
MyCloneable Object
(
    [object1] => SubObject Object
        (
            [instance] => 3
        )

    [object2] => SubObject Object
        (
            [instance] => 2
        )

)

It is possible to access a member of a freshly cloned object in a single expression:

Access member of freshly cloned object

<?php
$dateTime = new DateTime();
echo (clone $dateTime)->format('Y');
?>

The above example will output something similar to:

2016

Cloning with property overrides

As of PHP 8.5.0, clone is also available as a function, whose second parameter withProperties takes an array mapping property names to the values the copy receives. Only the listed properties are overridden, every other property keeps the value it was copied with.

The overrides are applied after __clone() has run, and they honour the visibility of all properties. Readonly properties can be updated on the copy even if they were already set on the original object, which makes the function suited to methods returning a modified copy.

Cloning an object with property overrides

<?php
class Point {
    public function __construct(
        public readonly int $x,
        public readonly int $y,
    ) {}

    public function withX(int $x): static
    {
        return clone($this, ['x' => $x]);
    }
}

$p = new Point(1, 2);
$q = $p->withX(10);

echo $q->x, ' ', $q->y;
?>

The above example will output:

10 2