> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# PHP Readonly Class
A `readonly` class in PHP (introduced in PHP 8.2) is a class modifier that implicitly declares all of its instance properties as `readonly`. Once an object of this class is instantiated and its properties are initialized, the object enforces shallow immutability. This prevents the reassignment of the properties themselves, though if a property holds a mutable object, the internal state of that referenced object can still be modified. Any attempt to reassign a property after initialization will throw an `Error` exception.
```php theme={"dark"}
readonly class Configuration
{
public function __construct(
public string $host,
public int $port,
public stdClass $metadata,
) {}
}
$config = new Configuration('localhost', 8080, new stdClass());
// Throws Error: Cannot modify readonly property Configuration::$port
// $config->port = 9000;
// Valid: Shallow immutability allows modification of internal state of mutable objects
$config->metadata->environment = 'production';
```
## Core Mechanics and Constraints
Applying the `readonly` modifier to a class enforces a strict set of rules at compile-time and runtime:
**1. Implicit Property Modification**
Every declared instance property within the class automatically inherits the `readonly` modifier. You do not need to explicitly declare individual properties as `readonly`.
**2. Strict Typing Requirement**
All properties within a `readonly` class must have an explicit type declaration. Untyped properties will trigger a fatal error. If a property can accept any value, it must be explicitly typed as `mixed`.
**3. Static Properties Forbidden**
A `readonly` class cannot declare `static` properties. The `readonly` modifier applies exclusively to instance properties, and attempting to define a static property within a `readonly` class will result in a fatal compile-time error.
**4. Dynamic Properties Forbidden**
Dynamic properties are strictly prohibited on `readonly` classes. Additionally, a `readonly` class cannot be marked with the `#[AllowDynamicProperties]` attribute. Attempting to apply this attribute will trigger a fatal error.
**5. Inheritance Restrictions**
The `readonly` state dictates strict inheritance rules. A `readonly` class can only be extended by another `readonly` class. Conversely, a standard (non-readonly) class cannot be extended by a `readonly` class.
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