__clone for object copying in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of cloning with __clone method

What is the output of this PHP code when cloning an object with a __clone method that modifies a property?

PHP

<?php
class Box {
    public $size;
    public function __construct($size) {
        $this->size = $size;
    }
    public function __clone() {
        $this->size *= 2;
    }
}
$box1 = new Box(5);
$box2 = clone $box1;
echo $box1->size . ',' . $box2->size;
?>

A 5,5

B 10,5

C 5,10

D 10,10

💡 Hint

Remember that __clone is called only on the cloned object, not the original.

The original object keeps its size 5. The cloned object's __clone doubles its size to 10. So output is '5,10'.

🧠 Conceptual

intermediate

Effect of __clone on nested objects

If an object has a property that is another object, what happens to that nested object when the outer object is cloned using __clone?

A The nested object is removed from the clone.

B The nested object is also cloned automatically.

C The nested object is converted to a string in the clone.

D The nested object is shared between original and clone unless manually cloned in __clone.

💡 Hint

Think about shallow vs deep copy.

By default, cloning an object copies its properties shallowly. Nested objects remain shared unless __clone explicitly clones them.

🔧 Debug

advanced

Why does cloning not duplicate nested objects?

Given this code, why does changing the nested object's property in the clone affect the original's nested object?

PHP

<?php
class Inner {
    public $value;
    public function __construct($v) {
        $this->value = $v;
    }
}
class Outer {
    public $inner;
    public function __construct() {
        $this->inner = new Inner(10);
    }
    public function __clone() {
        // no cloning of inner
    }
}
$a = new Outer();
$b = clone $a;
$b->inner->value = 20;
echo $a->inner->value;
?>

A Because PHP does not allow cloning nested objects.

B Because __clone does not clone the nested Inner object, so both share the same Inner instance.

C Because the Outer class constructor is called again on clone.

D Because the Inner object is immutable.

💡 Hint

Check what __clone does with nested objects.

The __clone method is empty, so the nested Inner object is not cloned. Both Outer objects share the same Inner instance, so changes affect both.

📝 Syntax

advanced

Identify the syntax error in __clone method

Which option contains a syntax error in the __clone method?

PHP

class Sample {
    public $data;
    public function __clone() {
        // code here
    }
}

A public function __clone { $this->data = clone $this->data; }

B public function __clone() { $this->data = clone $this->data; }

C public function __clone() { $this->data = $this->data; }

D } ;atad>-siht$ enolc = atad>-siht$ { )(enolc__ noitcnuf cilbup

💡 Hint

Check the function declaration syntax carefully.

Option A misses parentheses after __clone, causing a syntax error.

🚀 Application

expert

How to implement deep cloning with __clone

Given a class with multiple nested objects, how should you implement __clone to ensure a deep copy of all nested objects?

A Manually clone each nested object inside __clone, e.g., $this->nested = clone $this->nested; for each property.

B Assign nested objects to null inside __clone to avoid sharing.

C Use PHP's built-in deep clone function automatically called on clone.

D Do nothing; PHP clones nested objects by default.

💡 Hint

PHP cloning is shallow by default.

PHP only performs shallow cloning. To deep clone, you must manually clone each nested object inside __clone.