====== PHP RFC: #[\Deprecated] Attribute ======

  * Version: 1.4
  * Date: 2024-05-04
  * Author: Benjamin Eberlei, Tim Düsterhus 
  * Status: Under Discussion
  * First Published at: http://wiki.php.net/rfc/deprecated_attribute

===== Introduction =====

PHP’s internal functions and (class-)constants can be marked as deprecated, making this information available to Reflection and emitting deprecation errors (<php>E_DEPRECATED</php>), but there is no //equivalent// functionality for functions defined in userland.

While the functionality can be //emulated// using <php>trigger_error()</php> to emit a <php>E_USER_DEPRECATED</php> error when calling a deprecated function, combined with either parsing a doc comment for the <php>/** @deprecated */</php> annotation or attaching a custom-defined attribute and reading it with Reflection, it requires special handling depending on whether the function in question is an internal or a userland function. <php>ReflectionFunctionAbstract::isDeprecated()</php> always returns <php>false</php> for userland functions - and doc comments / custom attributes are unavailable for internal functions. For class constants the deprecation behavior cannot be emulated at all.

===== Proposal =====

A new attribute <php>#[\Deprecated]</php> shall be added:


<PHP>
#[Attribute(Attribute::TARGET_METHOD|Attribute::TARGET_FUNCTION|Attribute::TARGET_CLASS_CONSTANT)]
final class Deprecated
{
    public readonly ?string $message;

    public function __construct(?string $message = null, ?string $since = null) {}
}
</PHP>

Applying this attribute to a userland function, method, or class constant will add the internal ''ZEND_ACC_DEPRECATED'' flag to the element, resulting in a behavior that is consistent with the existing deprecation functionality for internal functions and class constants. This means:

  * For functions and methods <php>ReflectionFunctionAbstract::isDeprecated()</php> will return <php>true</php>.
  * For class constants <php>ReflectionClassConstant::isDeprecated()</php> will return <php>true</php> (this method is newly added in PHP 8.4).
  * Calling a function will emit a <php>E_USER_DEPRECATED</php> error (internal functions emit <php>E_DEPRECATED</php>, but this error code is reserved for internal functions).
  * Accessing a class constant will emit a <php>E_USER_DEPRECATED</php> error (likewise internal class constants use <php>E_DEPRECATED</php>).

The <php>$message</php> given within the attribute definition will be included in the error message when calling the function or accessing the class constant. It goes without saying that it will also be available to static analysis tools, just like the parameters of any other attribute.

The optional <php>$since</php> argument is appended to the error message. It signals the version since the function/method is deprecated. This is included because due its pre-existing use in internals, php-doc, Symfony and others. 

While internal, non-class constants can also be marked as deprecated, there is no support for attributes on them yet, and <php>ReflectionConstant</php> has just been added in PHP 8.4. Adding attribute support for non-class constants is out of scope of this RFC/PR for now.

Enum cases are internally implemented as class constants. Adding the <php>#[\Deprecated]</php> attribute to an enum case will behave as expected.

==== Examples ====

<PHP>
<?php

#[\Deprecated]
function test() {
}

#[\Deprecated("use test() instead")]
function test2() {
}

#[\Deprecated("use test() instead", since: "2.4")]
function test3() {
}

class Clazz {
    #[\Deprecated]
    public const OLD_WAY = 'foo';
    
    #[\Deprecated]
    function test() {
    }

    #[\Deprecated("use test() instead")]
    function test2() {
    }
}

enum MyEnum {
    #[\Deprecated]
    case OldCase;
}

test(); // Deprecated: Function test() is deprecated in test.php on line 29
test2(); // Deprecated: Function test2() is deprecated, use test() instead in test.php on line 30
test3(); // Deprecated: Function test2() is deprecated since 2.4, use test() instead in test.php on line 30
call_user_func("test"); // Deprecated: Function test() is deprecated in test.php on line 31

$cls = new Clazz();
$cls->test(); // Deprecated: Method Clazz::test() is deprecated in test.php on line 34
$cls->test2(); // Deprecated: Method Clazz::test2() is deprecated, use test() instead in test.php on line 35
Clazz::OLD_WAY; // Deprecated: Constant Clazz::OLD_WAY is deprecated in test.php on line 36

MyEnum::OldCase; // Deprecated: Enum case MyEnum::OldCase is deprecated in test.php on line 38

call_user_func([$cls, "test"]); // Deprecated: Method Clazz::test() is deprecated in test.php on line 40

?>
</PHP>

<PHP>
<?php

#[\Deprecated]
function test() {
}

$r = new ReflectionFunction('test');

var_dump($r->isDeprecated()); // bool(true)

?>
</PHP>

<PHP>
<?php

class Clazz {
    #[\Deprecated]
    public const OLD_WAY = 'foo';
}

$r = new ReflectionClassConstant(Clazz::class, 'OLD_WAY');

var_dump($r->isDeprecated()); // bool(true)

?>
</PHP>

<PHP>
<?php

#[\Deprecated]
function test1() {
}

#[\Deprecated()]
function test2() {
}

#[\Deprecated("use test() instead")]
function test3() {
}

$reflection = new ReflectionFunction('test1');
var_dump($reflection->getAttributes()[0]->newInstance());
/*
object(Deprecated)#3 (1) {
  ["message"]=>
  NULL
}
*/

$reflection = new ReflectionFunction('test2');
var_dump($reflection->getAttributes()[0]->newInstance());
/*
object(Deprecated)#2 (1) {
  ["message"]=>
  NULL
}
*/

$reflection = new ReflectionFunction('test3');
var_dump($reflection->getAttributes()[0]->newInstance());
/*
object(Deprecated)#1 (1) {
  ["message"]=>
  string(18) "use test() instead"
}
*/

?>
</PHP>

Further examples are given by [[https://github.com/php/php-src/pull/11293/files?file-filters%5B%5D=.phpt&show-viewed-files=true|the newly added tests within the PR for this RFC]].

===== Backward Incompatible Changes =====

<php>Deprecated</php> can no longer be used as a class name in the global namespace. A GitHub search for ''"class Deprecated " language:php symbol:deprecated'' revealed a total of 173 matches in source code. The vast majority of them appear to be defined within a namespace.

===== Proposed PHP Version(s) =====

Next minor (PHP 8.4).

===== RFC Impact =====

==== To SAPIs ====

None.

==== To Existing Extensions ====

The <php>#[\Deprecated]</php> attribute will also be available to internal functions and internal class constants. Within a stub file it will have the same effect as adding a <php>/** @deprecated */</php> doc comment. The attribute will //not// be automatically applied to existing functions having the doc comment, but extension authors are encouraged to apply the attribute for consistency reasons.

For extensions that are part of php-src the attribute will replace the existing doc comment as part of this RFC.

==== To Opcache ====

None.

==== New Constants ====

None.

==== php.ini Defaults ====

None.

===== Open Issues =====

A few things tracked in https://github.com/php/php-src/pull/11293

===== Future Scope =====

  * Supporting <php>#[\Deprecated]</php> on other targets of attributes that to not yet support deprecations for internally defined symbols.
  * Adding further metadata to the <php>#[\Deprecated]</php> attribute beyond a custom message (e.g. hints for replacements that IDEs could use).

===== Proposed Voting Choices =====

<doodle title="Implement the #[\Deprecated] attribute as described?" auth="timwolla" voteType="single" closed="false" closeon="2022-01-01T00:00:00Z">
   * Yes
   * No
</doodle>

===== Patches and Tests =====

https://github.com/php/php-src/pull/11293

===== Implementation =====

n/a

===== References =====

  * Implementation: https://github.com/php/php-src/pull/11293
  * Early Mailing List Discussion: https://externals.io/message/112554#112554

===== Rejected Features =====

  * Changes to the runtime behavior of deprecated functions and class constants are out of scope of this RFC (i.e. not emitting the <php>E_DEPRECATED</php> error for internal functions).
  * Making the <php>Deprecated</php> attribute class non-final: Child classes of attributes are not understood by the engine for technical reasons and the semantics of a child class would be less clear for static analysis tools.