Value objects are immutable objects that represent a set of values. They are used to encapsulate data and behavior, and are often used in domain-driven design.

Characteristics of Value Objects:

• Immutable (cannot be changed once created)
• Equal by value (not by reference)
• No identity (no unique ID)
• Can be used as array keys or hash map keys

Mutability:

Mutability refers to the ability of an object to change its state after creation.

Types of Mutability:

• Mutable: Can change state after creation (e.g., arrays, objects with setters)
• Immutable: Cannot change state after creation (e.g., value objects, strings)

Best Practices:

• Use immutable objects when possible (e.g., value objects, data transfer objects)
• Use mutable objects when necessary (e.g., entities, domain objects)
• Avoid mixing mutable and immutable objects in the same class

<?php 

class Money {
  private $amount;
  private $currency;

  public function __construct($amount, $currency) {
    $this->amount = $amount;
    $this->currency = $currency;
  }

  public function getAmount() {
    return $this->amount;
  }

  public function getCurrency() {
    return $this->currency;
  }

  public function equals(Money $other) {
    return $this->amount === $other->amount && $this->currency === $other->currency;
  }
}

Benefits of Immutable Objects:

• Thread-safety
• Easy to reason about and test
• Can be used as cache keys or hash map keys
• Reduces side effects and bugs

By using value objects and controlling mutability, we can write more predictable, maintainable, and scalable code in PHP OOP.