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22. Readonly Properties

Immutable object state.

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PHP 8.1 added the readonly modifier for typed properties, allowing them to be initialized once (typically in the constructor) and never modified afterward. Attempting to reassign a readonly property throws an Error.

Readonly properties are commonly combined with constructor promotion to build immutable value objects and DTOs concisely.

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Check your understanding

1. What happens when you try to modify a readonly property after initialization?
It silently ignores the writeIt throws an ErrorIt creates a new objectIt works normally
2. readonly properties are most useful for building what?
Mutable cachesImmutable value objectsGlobal stateLoops

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