{"id":2864,"date":"2021-11-26T02:07:20","date_gmt":"2021-11-26T02:07:20","guid":{"rendered":"https:\/\/phptutorial.net\/?page_id=2864"},"modified":"2025-04-07T11:21:44","modified_gmt":"2025-04-07T11:21:44","slug":"php-readonly-properties","status":"publish","type":"page","link":"https:\/\/www.phptutorial.net\/php-oop\/php-readonly-properties\/","title":{"rendered":"PHP Readonly Properties"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the PHP readonly properties that can be only initialized once.<\/p>\n\n\n\n

Introduction to the PHP readonly properties #<\/a><\/h2>\n\n\n\n

PHP 8.1 introduced the readonly class properties. The readonly properties allow you to define properties that can be only initialized once within the class. <\/p>\n\n\n\n

To define a readonly property, you use the readonly<\/code> keyword in a typed property<\/a>:<\/p>\n\n\n

<?php<\/span>\n\n\nclass<\/span> MyClass<\/span>\n<\/span>{\n    private<\/span> readonly type propertyName;\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Or<\/p>\n\n\n
<?php<\/span>\n\n\nclass<\/span> MyClass<\/span>\n<\/span>{\n    public<\/span> function<\/span> __construct<\/span>(private readonly type propertyName)<\/span>\n    <\/span>{\n    }\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
For example, the following defines a User<\/code> class with the $username<\/code> as a readonly property:<\/p>\n\n\n
<?php<\/span>\n\n\nclass<\/span> User<\/span>\n<\/span>{\n    public<\/span> readonly string $username;\n\n    public<\/span> function<\/span> __construct<\/span>(string $username)<\/span>\n    <\/span>{\n        $this<\/span>->username = $username;\n    }\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, you can only initialize the $username<\/code> once when creating a new User<\/code> object like this:<\/p>\n\n\n
$user = new<\/span> User('joe'<\/span>,'secure'<\/span>);<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
If you attempt to change the $username<\/code> property after that, you’ll get an error:<\/p>\n\n\n
$user->username = 'john'<\/span>;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
Fatal error: Uncaught Error: Cannot modify readonly property User::$username<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The following example also causes an error because it attempts to initialize the $username<\/code> property from the outside of the User class:<\/p>\n\n\n
<?php<\/span>\n\n\nclass<\/span> User<\/span>\n<\/span>{\n    public<\/span> readonly string $username;\n}\n\n$user = new<\/span> User();\n$user->username = 'joe'<\/span>;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
Fatal error: Uncaught Error: Cannot initialize readonly property User::$username from global scope<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
To fix it, you can add a constructor like the above example. Alternatively, you can define a method to initialize the readonly property from within the class as follows:<\/p>\n\n\n
<?php<\/span>\n\n\nclass<\/span> User<\/span>\n<\/span>{\n    public<\/span> readonly string $username;\n    public<\/span> string $password;\n\n    public<\/span> function<\/span> setUsername<\/span>(string $username)<\/span>: void<\/span>\n    <\/span>{\n        $this<\/span>->username = $username;\n    }\n    \n}\n\n$user = new<\/span> User();\n$user->setUsername('joe'<\/span>);\n\necho<\/span> $user->username;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that if you call the setUsername()<\/code> method a second time, you’ll get an error because it modifies to the username property that was already initialized.<\/p>\n\n\n
<?php<\/span>\n\n\nclass<\/span> User<\/span>\n<\/span>{\n    public<\/span> readonly string $username;\n    public<\/span> string $password;\n\n    public<\/span> function<\/span> setUsername<\/span>(string $username)<\/span>: void<\/span>\n    <\/span>{\n        $this<\/span>->username = $username;\n    }\n    \n}\n\n$user = new<\/span> User();\n$user->setUsername('joe'<\/span>);\n$user->setUsername('john'<\/span>);<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
Fatal error: Uncaught Error: Cannot modify readonly property User::$username<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Readonly & typed properties #<\/a><\/h2>\n\n\n\n
A property without a type has the default value of null<\/code>. For example:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> User<\/span>\n<\/span>{\n    public<\/span> $username;\n}\n\n$user = new<\/span> User();\necho<\/span> $user->username;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
NULL<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the value of the $username<\/code> property is null<\/code> by default until you assign a value to it.<\/p>\n\n\n\n
Because of this, PHP only supports readonly on a typed property. If you attempt to use the readonly<\/code> keyword with a property without a type, you’ll get an error. For example:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> User<\/span>\n<\/span>{\n    public<\/span> readonly $username;\n}\n<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
Fatal error: Readonly property User::$username must have type<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Mutability #<\/a><\/h2>\n\n\n\n
A readonly property doesn’t ensure the immutability of objects. Let’s see the following example.<\/p>\n\n\n\n
First, define a UserProfile<\/code> class that has two properties name<\/code> and phone<\/code>:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> UserProfile<\/span>\n<\/span>{\n    public<\/span> function<\/span> __construct<\/span>(private string $name, private string $phone)<\/span>\n    <\/span>{\n    }\n    public<\/span> function<\/span> changePhone<\/span>(string $phone)<\/span>\n    <\/span>{\n        $this<\/span>->phone = $phone;\n    }\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, define the User<\/code> class that has a property whose type is an object of the UserProfile<\/code> class:<\/p>\n\n\n
class<\/span> User<\/span>\n<\/span>{\n    private<\/span> readonly string $username;\n    private<\/span> readonly UserProfile $profile;\n\n    public<\/span> function<\/span> __construct<\/span>(string $username)<\/span>\n    <\/span>{\n        $this<\/span>->username = $username;\n    }\n\n    public<\/span> function<\/span> setProfile<\/span>(UserProfile $profile)<\/span>\n    <\/span>{\n        $this<\/span>->profile = $profile;\n    }\n\n    public<\/span> function<\/span> profile<\/span>()<\/span>: UserProfile<\/span>\n    <\/span>{\n        return<\/span> $this<\/span>->profile;\n    }\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, create a User<\/code> object and assign a profile to it:<\/p>\n\n\n
$user = new<\/span> User('joe'<\/span>);\n$user->setProfile(new<\/span> UserProfile('Joe Doe'<\/span>,'(408)-555-6666'<\/span>));<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The $profile<\/code> is a readonly property of the User<\/code> class. And you can initialize it once. However, you can change the properties of a readonly property like this:<\/p>\n\n\n
$user->profile()->changePhone('(408)-999-9999'<\/span>);\nvar_dump($user->profile());<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
object(UserProfile)#2 (2) {\n  [\"name\":\"UserProfile\":private]=>  string(7) \"Joe Doe\"\n  [\"phone\":\"UserProfile\":private]=>  string(14) \"(408)-999-9999\"\n}<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
Put it all together:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> UserProfile<\/span>\n<\/span>{\n    public<\/span> function<\/span> __construct<\/span>(private string $name, private string $phone)<\/span>\n    <\/span>{\n    }\n    public<\/span> function<\/span> changePhone<\/span>(string $phone)<\/span>\n    <\/span>{\n        $this<\/span>->phone = $phone;\n    }\n}\n\nclass<\/span> User<\/span>\n<\/span>{\n    private<\/span> readonly string $username;\n    private<\/span> readonly UserProfile $profile;\n\n    public<\/span> function<\/span> __construct<\/span>(string $username)<\/span>\n    <\/span>{\n        $this<\/span>->username = $username;\n    }\n\n    public<\/span> function<\/span> setProfile<\/span>(UserProfile $profile)<\/span>\n    <\/span>{\n        $this<\/span>->profile = $profile;\n    }\n\n    public<\/span> function<\/span> profile<\/span>()<\/span>: UserProfile<\/span>\n    <\/span>{\n        return<\/span> $this<\/span>->profile;\n    }\n}\n\n\n$user = new<\/span> User('joe'<\/span>);\n$user->setProfile(new<\/span> UserProfile('Joe Doe'<\/span>,'(408)-555-6666'<\/span>));\n$user->profile()->changePhone('(408)-999-9999'<\/span>);\nvar_dump($user->profile());<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • A readonly property can be initialized once from within the class.<\/li>\n\n\n\n
  • Use the readonly<\/code> keyword in a typed property to make the property readonly.<\/li>\n<\/ul>\n
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    \n\t\t
    Did you find this tutorial useful?<\/div>\n\t\t
    \n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t
    \n\t\t
    \n\t\t\t
    <\/div>\n\t\t\t\n\t\t\t
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
PHP only allows you to initialize the $username<\/code> property from within the class itself, either from the constructor<\/a> or a method.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n