PHP 8.1 introduced readonly properties to simplify creating immutable objects. Readonly properties can only be assigned once, either at declaration or in the constructor, preventing further modifications. Before this feature, developers enforced immutability manually using private properties, getters, and no setters. Now, PHP itself enforces immutability via readonly. 1. Readonly prevents accidental changes. 2. It clearly communicates code intent. 3. It reduces boilerplate for encapsulation. However, readonly only protects the property reference, not nested data—arrays and objects can still change internally. For deep immutability, use value objects or immutable collections. Best practices include combining readonly with final classes, named constructors, and immutable types. Thus, readonly is a powerful tool for predictable object design but not a complete solution for nested mutability.

PHP 8.1 introduced readonly properties, which make it much easier to create immutable objects—objects whose state can't be modified after creation. Before this feature, developers had to manually enforce immutability by using private properties with no setters and initializing them through the constructor. Now, readonly does most of that work automatically.

What Are Readonly Properties?

In PHP 8.1 , a readonly property can only be assigned once—either at declaration or in the constructor. After that, any attempt to modify it will result in an error.

This restriction aligns perfectly with the concept of immutability, where an object’s internal state is fixed from the moment it's created.

Example:

class User {
    public function __construct(
        public readonly string $name,
        public readonly int $age
    ) {}
}

Once you create a User instance, $name and $age can’t be changed:

$user = new User('Alice', 30);
$user->name = 'Bob'; // throws Error: Cannot modify readonly property

How Readonly Helps Enforce Immutability

Before PHP 8.1, enforcing immutability required boilerplate code like:

• Private properties
• Public getters
• No setters
• Manual checks in methods

Now, with readonly, the language itself enforces that behavior. You don’t have to write extra code to prevent mutation—you just declare the property as readonly.

Here’s what makes it helpful:

• ? Prevents accidental changes
• ? Communicates intent clearly in code
• ? Reduces boilerplate for encapsulation

Also, because the restriction is enforced at runtime, it catches bugs early and helps maintain consistency across your app.

Limitations and Gotchas

While readonly is great for immutability, there are some things to keep in mind:

• It only protects the property reference, not nested data:
  If a readonly property is an array or an object, its contents can still change.

  class Data {
      public function __construct(
          public readonly array $items = []
      ) {}
  }
  
  $d = new Data([1,2]);
  $d->items[] = 3; // allowed! The array itself is mutable.

• You can’t reassign the property, but internal state might still change:
  For true deep immutability (e.g., with objects), you’ll need to use value objects or immutable collections.

So while readonly ensures the property itself isn’t reassigned, it doesn’t fully lock down complex types inside it.

Best Practices When Using Readonly

If you're building immutable objects, here are a few tips to make the most of readonly:

• Use it with value objects, where identity is based on data, not reference.
• Combine with final classes if you want to prevent subclassing that could break immutability.
• Consider using named constructors for more expressive object creation.
• Pair with immutable collections or other immutable types when possible.

For example:

final class Point {
    public function __construct(
        public readonly int $x,
        public readonly int $y
    ) {}
}

This way, you’re signaling both to PHP and to other developers that this object shouldn’t—and can’t—change.

Basically, readonly in PHP 8.1 is a powerful tool for creating immutable objects without writing extra boilerplate. It’s not a silver bullet for all mutability issues, especially with nested data, but it's a solid foundation that makes clean, predictable object design much easier.

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