In PHP, when you assign an object to another PHP variable, you're not actually creating a copy of the object. Instead, both variables reference the same object in memory. This means that changes made to one variable affect the other. If you need an independent copy of an object, PHP provides object cloning using the clone keyword. In this article, we’ll explore PHP object cloning in detail, understand shallow and deep copies, and learn how to use the __clone() method effectively.

Understanding Object References in PHP

Before diving into cloning, it’s crucial to understand how objects are stored in PHP. Unlike primitive data types (like integers and strings), objects are stored by reference. This means when you assign an object to a new variable, you’re not creating a copy — both variables point to the same object in memory.

Many PHP developers confuse object references with object comparison in PHP, assuming that two objects with the same values are the same instance. However, references deal with how objects are stored, while comparisons determine equality based on values or identity. Understanding this difference is key before we move on to cloning.

class Example {

    public $data;

}

$original = new Example();

$original->data = "Hello";

$copy = $original; // Assigning reference, not a new object

$copy->data = "World";

echo $original->data; // Output: World (Both variables point to the same object)

As you can see, $copy and $original reference the same memory location. To create an actual copy of the object, we use cloning.

Cloning Objects with clone

PHP allows object cloning using the clone keyword. This creates a shallow copy of the object, meaning the object itself is duplicated, but any properties that are objects remain referenced.

class Example {

    public $data;

}

$original = new Example();

$original->data = "Hello";

$copy = clone $original; // Creates a new object with the same property values

$copy->data = "World";

echo $original->data; // Output: Hello (Original object remains unchanged)

Now, $copy is an independent object with its own memory allocation.

Shallow Copy vs Deep Copy

Shallow Copy (Default Cloning Behavior)

When you clone an object in PHP, the default behavior is to copy all properties as they are. However, if a property holds a reference to another object, the cloned object will still point to the same referenced object.

class Inner {

    public $value;

}

class Outer {

    public $inner;

}

$original = new Outer();

$original->inner = new Inner();

$original->inner->value = "Shared";

$copy = clone $original;

$copy->inner->value = "Modified";

echo $original->inner->value; // Output: Modified (Both objects share the same reference)

Deep Copy (Creating a Truly Independent Copy)

To create a deep copy, we need to explicitly clone inner objects inside the __clone() method.

class Inner {

    public $value;

}

class Outer {

    public $inner;

    public function __clone() {

        $this->inner = clone $this->inner; // Cloning inner object

    }

}

$original = new Outer();

$original->inner = new Inner();

$original->inner->value = "Shared";

$copy = clone $original;

$copy->inner->value = "Modified";

echo $original->inner->value; // Output: Shared (Now they are separate objects)

Now, $copy->inner is completely independent of $original->inner.

When to Use Object Cloning

When Cloning is Useful

  • Creating a backup state of an object before modifying it.

  • Using the prototype design pattern, where object duplication is required.

  • Cloning configuration objects that need independent modifications.

When Cloning Should Be Avoided

  • When objects hold database connections or external resources, as this can lead to unexpected behaviors.

  • When the cloned object requires re-initialization, in which case manually instantiating a new object is better.

Object Cloning vs. Creating a New Object

Feature

Cloning an Object

Creating a New Object

Instance

Duplicates an existing object

Creates a fresh instance

Performance

Faster when reusing complex objects

Slightly slower

Object References

Can retain references in a shallow copy

Always independent

Best Practices for Object Cloning

  • Always override __clone() if an object contains references to other objects.

  • Avoid cloning large objects unnecessarily to improve performance.

  • Consider using factory methods for controlled object duplication.

Conclusion

PHP object cloning is a powerful feature that allows efficient duplication of objects while maintaining reference integrity. Understanding the difference between shallow and deep copies ensures that your cloned objects behave as expected. Always use the __clone() method wisely to avoid unwanted side effects.

By mastering object cloning, you can write more efficient and bug-free PHP applications!

Further Reading: