PHP 8.1引入了只讀屬性，用于聲明初始化后不可更改的類屬性。開發(fā)者可在構造函數(shù)或聲明時初始化只讀屬性，之后無法修改，適用于需確保數(shù)據(jù)完整性的場景，如實體、DTO和配置值。使用時需注意：只讀屬性不能在構造函數(shù)外賦值，不能與var或無訪問修飾符一起使用，且對數(shù)組或對象僅防止重新賦值，不阻止內部狀態(tài)變更。不適合需要頻繁更新屬性或性能敏感的場景。常見用法包括：1）領域驅動設計中的實體ID；2）API響應的數(shù)據(jù)傳輸對象；3）加載后不應變動的配置項。局限性包括：1）構造后不可重新賦值；2）不支持var關鍵字；3）不阻止對象或數(shù)組內容變更。

In PHP 8.1, readonly properties are a new feature that allows developers to declare class properties that cannot be modified after being initialized. This means once you set the value of a readonly property during object creation or within the constructor, it remains fixed and cannot be changed later in the code.

This is especially useful when working with value objects or entities where immutability is desired — for example, mapping database records or API responses where fields like IDs or timestamps shouldn’t change after being set.

How to Declare Readonly Properties

To declare a readonly property, you simply add the readonly modifier before the property declaration in a class:

class User {
    public function __construct(
        private readonly string $id,
        private readonly string $name
    ) {}
}

You can also declare them individually inside the class body if you're not using the constructor promotion syntax:

class Product {
    public readonly string $sku;

    public function __construct(string $sku) {
        $this->sku = $sku;
    }
}

A few key things to know:

• Readonly properties must be initialized either in the declaration or in the constructor.
• You can read them from anywhere (inside or outside the class), but you can't write to them once set.
• They can be used with any visibility: public, protected, or private.

Common Use Cases

Readonly properties are ideal for scenarios where data integrity is important. Here are a few practical examples:

• Entities in domain-driven design: For example, an Order entity might have a readonly orderId to ensure it doesn’t accidentally change during processing.
• Data transfer objects (DTOs): These often represent structured data coming from APIs or databases, where fields should stay constant after parsing.
• Configuration values: Once loaded from a config file, certain settings might need to remain unchanged throughout the application lifecycle.

These use cases benefit from the clarity and safety that readonly properties provide.

Limitations and Things to Watch Out For

While readonly properties are powerful, there are a few limitations and gotchas to keep in mind:

• You cannot reassign the property anywhere after construction — not even inside the class methods.
• You can't use readonly on properties declared with var or without visibility keywords.
• It only applies to simple variables — if the property is an object or array, its internal state can still change unless you deeply protect it manually.

For example:

class Example {
    public function __construct(
        public readonly array $data
    ) {}
}

$ex = new Example(['tags' => ['a', 'b']]);
$ex->data['tags'][] = 'c'; // This is allowed!

So while $data itself can't be replaced, its contents can still be modified.

When Not to Use Them

Although readonly properties help enforce immutability, they aren’t always the right choice:

• If your class needs to update certain properties during its lifecycle (like status flags or counters), readonly won’t work.
• In performance-sensitive contexts where you’re instantiating many objects and want flexibility in updating properties without creating new instances.

In these cases, standard mutable properties are better suited.

So yeah, readonly properties in PHP 8.1 give you a clean way to define immutable class members — great for data models where consistency matters. Just remember to use them where appropriate, and watch out for their limitations.基本上就這些。

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