PHP RFC: Deprecate dynamic properties

• Date: 2021-08-23
• Author: Nikita Popov nikic@php.net
• Status: Under Discussion
• Target Version: PHP 8.2
• Implementation: https://github.com/php/php-src/pull/7390

When writing to a property that has not been declared, PHP will silently create a dynamic property instead. In modern code, this is rarely done intentionally. This RFC proposes to deprecate and later remove the creation of dynamic properties. stdClass and __get/__set are not affected by this change.

class User {
    public $name;
}
 
$user = new User;
 
// Assigns declared property User::$name.
$user->name = "foo";
 
// Oops, a typo:
$user->nane = "foo";
// PHP <= 8.1: Silently creates dynamic $user->nane property.
// PHP    8.2: Raises deprecation warning, still creates dynamic property.
// PHP    9.0: Throws Error exception.

“Dynamic property” here refers to a property that has not been declared in the class. It has no relation to the access style (e.g. $user->{'na' . 'me'} is still an access to a declared property).

The creation of dynamic properties on classes that don't inherit from stdClass is deprecated in PHP 8.2 and becomes an Error exception in PHP 9.0. All used properties should be declared in the class declaration.

class Foo {}
$foo = new Foo;
 
// Deprecated: Creation of dynamic property Foo::$bar is deprecated
$foo->bar = 1;
 
// No deprecation warning: Dynamic property already exists.
$foo->bar = 2;

Objects of type stdClass and inheriting classes continue to support dynamic properties.

$obj = (object) []; // = new stdClass;
 
// No deprecation warning
$obj->foo = 1;
 
class myStdClass extends stdClass {}
$obj2 = new myStdClass;
 
// No deprecation warning
$obj2->bar = 1;

stdClass objects are specifically intended to hold dynamic properties. extends stdClass is offered as a simple migration strategy for custom classes that are also specifically intended for use with dynamic properties.

It should be noted that properties accessed through __get()/__set() are not considered as “dynamic properties”. The following example does not generate any deprecation warnings:

class ArrayLikeObject {
    private array $data = [];
    public function &__get($name) { return $this->data[$name]; }
    public function __isset($name, $value) { return isset($this->data[$name]; }
    public function __set($name, $value) { $this->data[$name] = $value; }
    public function __unset($name) { unset($this->data[$name]; }
}
 
$obj = new ArrayLikeObject;
 
// Calls ArrayLikeObject::__set(), no deprecation warning.
$obj->foo = 1;

Removing support for dynamic properties constitutes a significant backwards compatibility break. While modern PHP code consistently declares used properties, this is not necessarily the case for legacy code. When encountered with a dynamic property deprecation warning, there are a number of things that can be done to avoid it.

The simplest and most common would be to simply add the property declaration:

class Test {
    public $value; // <-- Add property declaration.
 
    public function __construct($value) {
        $this->value = $value;
    }
}

For classes that intentionally don't have a fixed set of properties, it's possible to either implement magic __get()/__set(), or to extend from the stdClass class, or from ArrayObject in ARRAY_AS_PROPS mode.

Using magic getters/setters provides the most control, but extending from stdClass will make dynamic property accesses more efficient by using optimized engine hooks. It will also match the current behavior most closely, for example with regard to the behavior of foreach or property_exists().

In some cases it is desirable to associate information with objects that you do not own. Previously, it was possible to add a dynamic property for this purpose. Instead, a WeakMap should be used to store the information in a non-intrusive way:

class Test {
    private WeakMap $extraInfo;
 
    public function addExtraInfo(object $obj) {
        // Instead of:
        $obj->extraInfo = ...;
        // Use:
        $this->extraInfo[$obj] = ...;
    }
}

In rare cases, dynamic properties are used for lazy initialization. For example Symfony's Constraint::$groups property is not declared and then dynamically created inside __get(). This use-case can be accommodated by declaring the property, but then unsetting it in the constructor:

abstract class Constraint {
    public $groups;
 
    public function __construct() {
        unset($this->groups);
    }
 
    public function __get($name) {
        // Will get called on first access, but once initialized.
        $this->groups = ...;
    }
}

A declared property that has been unset remains a declared property, and will not result in a dynamic property when it is reinitialized.

This RFC offers extends stdClass as a way to opt-in to the use of dynamic properties. Some people have suggested that we could use a magic marker interface (implements SupportsDynamicProperties), an attribute (#[SupportsDynamicProperties]) or a trait (use DynamicProperties;) instead.

The reasoning behind the extends stdClass choice is that it works without any additional special support: We definitely need to allow dynamic properties on stdClass, and following the Liskov substitution principle, child classes should inherit this behavior. As such, the extends stdClass escape hatch will work anyway, and the question is more whether we want to offer anything in addition to it.

Another way to view this is that stdClass could implement __get()/__set() to provide its “dynamic properties” support, in which case these methods would naturally be inherited. Of course, it currently doesn't do so, but it probably should once dynamic property support is removed.

Using an interface or attribute instead would require the engine to continue supporting dynamic properties on arbitrary classes long term, rather than simply inheriting the behavior from a single class that implements the functionality.

A trait based on __get()/__set() could be provided, and would be usable in multiple-inheritance cases where extending from stdClass is not possible:

class DynamicProperties {
    private array $dynamicProps = [];
    public function &__get($name) { return $this->dynamicProps[$name]; }
    public function __isset($name, $value) { return isset($this->dynamicProps[$name]; }
    public function __set($name, $value) { $this->dynamicProps[$name] = $value; }
    public function __unset($name) { unset($this->dynamicProps[$name]; }
}

However, such a trait would not differ from simply implementing these methods in userland. Unlike extends stdClass, it would not benefit from optimized internal hooks, and it would not be able to offer exactly the same functionality. A custom implementation does not take a significant amount of code, but has more control over what exactly it wants to support. For example, next to the above baseline implementation, a class might also want to implement __debugInfo() and the Traversable interface.

The Locked classes RFC took an alternative approach to this problem space: Rather than deprecating/removing dynamic properties and providing an opt-in for specific classes, it instead allowed marking specific classes as locked in order to forbid creation of dynamic properties on them.

I don't believe that this is the right strategy, because in contemporary code, classes being “locked” is the default state, while classes that require dynamic properties are a rare exception. Additionally, this requires that class owners (which may be 3rd party packages) consistently add the “locked” keyword to be effective.

An alternative that has been discussed in the context of the language evolution proposal is to instead disallow the use of dynamic properties in a file through a declare directive (whether that be a fine-grained option or an “edition” mechanism). This does not require the cooperation of 3rd-party libraries.

However, based on the discussion on the language evolution proposal, this would only delay the time where disallowed dynamic properties become the default and only behavior, as there was a strong consensus that diverging language behavior should not be maintained indefinitely. Dynamic properties would ultimately still get deprecated and removed.

Yes/No.