PHP: rfc:deprecated_interfaces

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====== PHP RFC: #[\Deprecated] for interfaces ======
  * Version: 0.1
  * Date: 2026-05-10
  * Author: Daniel Scherzer, daniel.e.scherzer@gmail.com
  * Status: Draft
  * Implementation: https://github.com/php/php-src/pull/21998
  * Discussion thread:

===== Introduction =====

The <php>#[\Deprecated]</php> attribute was introduced in PHP 8.4 ([[rfc:deprecated_attribute]]) and could be used to emit deprecation warnings when calling a function (or class method), or when accessing a class constant (or enum case). Starting in 8.5, deprecation warnings could also be emitted when accessing a global constant ([[rfc:attributes-on-constants]]) or when <php>use</php>ing a trait ([[rfc:deprecated_traits]]). This RFC proposes to add support for emitting deprecation warnings when an interface is extended or implemented.

<PHP>
<?php

#[\Deprecated]
trait DemoInterface {}

class DemoClass implements DemoInterface {}

// Reports:
// Deprecated: Interface DemoInterface implemented by DemoClass is deprecated in %s on line %d
?>
</PHP>

===== Proposal =====
The <php>#[\Deprecated]</php> attribute will be allowed on interfaces. The existing target validation when applied to classes or enums is maintained.

When the interfaces that a class implements are loaded, any deprecated interfaces emit a deprecation message; these can be converted to an exception via a user error handler, and an uncaught exception will be treated the same as trying to implement a non-interface in terms of registration behavior. The same applies to interfaces extending other interfaces if those other interfaces are marked as deprecated.

Interfaces only emit deprecation warnings when //directly// extended or implemented, implementing child interfaces or extending implementing classes does not result in extra errors. See the comparison with existing tools below.

==== Examples ====

Simple example:
<PHP>
<?php

#[\Deprecated]
trait DemoInterface {}

class DemoClass implements DemoInterface {}

// Reports:
// Deprecated: Interface DemoInterface implemented by DemoClass is deprecated in %s on line %d
?>
</PHP>

With an exception:
<PHP>
<?php

function my_error_handler(int $errno, string $errstr, ?string $errfile = null, ?int $errline = null) {
	throw new \ErrorException($errstr, 0, $errno, $errfile, $errline);
}

set_error_handler('my_error_handler');

#[\Deprecated]
interface DemoInterface {}

class DemoClass implements DemoInterface {}

// Reports:
// Fatal error: Uncaught ErrorException: Interface DemoInterface implemented by DemoClass is deprecated in %s:%d

?>
</PHP>

==== Tool comparison ====

Given the following code:
<PHP>
<?php

/**
 * @deprecated
 */
interface BaseInterface {}

interface ChildInterface extends BaseInterface {}

class ImplementsBase implements BaseInterface {}

class ImplementsBoth implements BaseInterface, ChildInterface {}

class ImplementsChild implements ChildInterface {}

class ExtendsBaseImplementor extends ImplementsBase {}

?>
</PHP>

existing static analysis tools emit warnings for different sets of declarations. The behavior of the tools and the engine is as follows, where "yes" means a warning is emitted (with the engine behavior coming from using <php>#[\Deprecated]</php> rather than a comment):

^ Declaration                                ^ Phan  ^ PHPStan  ^ Mago  ^ Psalm  ^ Engine  ^
| <php>interface ChildInterface...    </php> | yes   | yes      | yes   | no     | yes     |
| <php>class ImlementsBase...         </php> | yes   | yes      | no    | yes    | yes     |
| <php>class ImlementsBoth...         </php> | yes   | yes      | no    | yes    | yes     |
| <php>class ImlementsChild...        </php> | no    | no       | no    | yes    | no      |
| <php>class ExtendsBaseImplementor...</php> | no    | no       | no    | yes    | no      |


PHPStan is as of the version on the playground (https://phpstan.org/try) on 2026.05.10, Psalm is as of commit 5c1c31c which was on its playground (https://psalm.dev/) on the same date. Phan is as of version 6.0.5, Mago is as of version 1.26.0.

===== Backward Incompatible Changes =====
None

===== Proposed PHP Version(s) =====
Next PHP (8.6)

===== RFC Impact =====
==== To the Ecosystem ====
IDEs and static analyzers will likely want to update to surface deprecation messages when an interface is extended or implemented without waiting for PHP to emit the errors.

==== To Existing Extensions ====
This lays the engine support for deprecating PHP-provided interfaces, but none are currently marked as deprecated.

==== To SAPIs ====
No impact other than deprecations being emitted when a deprecated interface is used.

===== Open Issues =====
Make sure there are no open issues when the vote starts!

===== Future Scope =====
Other places that <php>#[\Deprecated]</php> could eventually be supported include

  * extending a class
  * overriding a method
  * accessing a property

But the support for deprecating the use of a interface is pretty straightforward and there isn't room for future improvements within the context of deprecating interface.


===== Voting Choices =====
<doodle title="Add support for #[\Deprecated] on interfaces?" voteType="single" closed="true" closeon="2025-08-07T23:59:59Z" auth="daniels">
   * Yes
   * No
</doodle>

===== Patches and Tests =====
https://github.com/php/php-src/pull/21998

===== Implementation =====
After the RFC is implemented, this section should contain:
  - the version(s) it was merged into
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature

===== References =====

  * [[rfc:deprecated_attribute]]
  * [[rfc:attributes-on-constants]]
  * [[rfc:deprecated_traits]]

===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.

===== Changelog =====
If there are major changes to the initial proposal, please include a short summary with a date or a link to the mailing list announcement here, as not everyone has access to the wikis' version history.

  * v0.1: created
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