__serialize and __unserialize in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of __serialize method in PHP

What is the output of the following PHP code when serialized?

PHP

<?php
class User {
    private string $name;
    private int $age;

    public function __construct(string $name, int $age) {
        $this->name = $name;
        $this->age = $age;
    }

    public function __serialize(): array {
        return ['name' => $this->name, 'age' => $this->age];
    }
}

$user = new User('Alice', 30);
print_r($user->__serialize());

A Array ( [name] => Alice [age] => 30 )

B Array ( [0] => Alice [1] => 30 )

C Array ( [name] => Alice )

D Array ( [age] => 30 )

💡 Hint

The __serialize method returns an associative array with keys and values representing the object's data.

The __serialize method returns an array with keys 'name' and 'age' holding the respective property values. So the output is an associative array with both keys and values.

❓ Predict Output

intermediate

Output of __unserialize method in PHP

What will be the output of the following PHP code?

PHP

<?php
class Product {
    public string $title;
    public float $price;

    public function __unserialize(array $data): void {
        $this->title = $data['title'];
        $this->price = $data['price'];
    }
}

$data = ['title' => 'Book', 'price' => 12.5];
$product = new Product();
$product->__unserialize($data);
echo $product->title . ' costs $' . $product->price;

A Undefined property: Product::$title

B Book costs $0

C Book costs $12.5

D Fatal error: Unserialize failed

💡 Hint

The __unserialize method sets object properties from the given array.

The __unserialize method assigns the 'title' and 'price' from the array to the object's properties. So echoing them prints 'Book costs $12.5'.

🔧 Debug

advanced

Identify the error in __serialize implementation

What error will this PHP code produce when trying to serialize the object?

PHP

<?php
class Person {
    private string $name;
    private int $age;

    public function __construct(string $name, int $age) {
        $this->name = $name;
        $this->age = $age;
    }

    public function __serialize(): array {
        return json_encode(['name' => $this->name, 'age' => $this->age]);
    }
}

$person = new Person('Bob', 25);
serialize($person);

A No error, serialization works fine

B TypeError: Return value of __serialize() must be of the type array, string returned

C Warning: json_encode() expects parameter 1 to be array, string given

D Fatal error: Cannot serialize private properties

💡 Hint

Check the return type of __serialize method.

The __serialize method must return an array, but here it returns a string from json_encode, causing a TypeError.

📝 Syntax

advanced

Which __unserialize method is syntactically correct?

Which option shows a syntactically correct __unserialize method in PHP?

A

public function __unserialize(array $data): void {
    $this->name = $data['name'];
    $this->age = $data['age'];
}

B

public function __unserialize($data) {
    $this->name = $data['name'];
    $this->age = $data['age'];
}

C

public function __unserialize(array $data) {
    $this->name = $data['name'];
    $this->age = $data['age'];
}

D

public function __unserialize(array $data): string {
    $this->name = $data['name'];
    $this->age = $data['age'];
    return 'done';
}

💡 Hint

Check the method signature and return type for __unserialize.

The __unserialize method must accept an array and return void. Option A matches this exactly. Option A lacks type hint, C lacks return type, A returns string which is invalid.

🚀 Application

expert

How many items in serialized data after __serialize?

Given this PHP class, how many key-value pairs will the __serialize method return?

PHP

<?php
class Session {
    private string $id;
    private array $data;
    private bool $active;

    public function __construct(string $id, array $data, bool $active) {
        $this->id = $id;
        $this->data = $data;
        $this->active = $active;
    }

    public function __serialize(): array {
        return [
            'id' => $this->id,
            'data' => $this->data,
            'active' => $this->active
        ];
    }
}

$session = new Session('abc123', ['user' => 'john'], true);
$serialized = $session->__serialize();

A 2

B 0

C 1

D 3

💡 Hint

Count the number of keys returned in the array from __serialize.

The __serialize method returns an array with three keys: 'id', 'data', and 'active'. So the count is 3.