Readonly Classes (PHP 8.2)

Also Known As

TL;DR

Explanation

PHP 8.2 extended the readonly keyword from individual properties to entire class declarations. A readonly class implicitly makes all typed, non-static properties readonly — they can only be initialised in the constructor. This is ideal for DTOs, value objects, and command/query objects. Untyped properties are not allowed in readonly classes. Cloning a readonly object with modified properties requires clone with property initialisation syntax (PHP 8.3+ adds clone with syntax). Combining with constructor property promotion gives maximally concise immutable objects.

Common Misconception

Why It Matters

Common Mistakes

• Trying to use readonly classes with properties that need to change after construction — they are for immutable objects.
• Not knowing that readonly classes were added in PHP 8.2 — using the syntax on 8.1 causes a parse error.
• Adding mutable static properties to readonly classes — static properties are not affected by readonly.
• Cloning readonly class instances hoping to modify properties — clone creates an identical immutable copy.

Code Examples

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

readonly class Point { public function __construct(
  public float $x,
  public float $y,
) {} } // readonly on class covers all properties

References

• ↗ https://www.php.net/manual/en/language.oop5.properties.php#language.oop5.properties.readonly-properties

Tags

Related Terms