PHP RFC: #[\Deprecated] Attribute

• Version: 1.3
• Date: 2024-04-15
• Author: Benjamin Eberlei, Tim Düsterhus
• Status: Under Discussion
• First Published at: http://wiki.php.net/rfc/deprecated_attribute

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

While the functionality can be emulated using trigger_error() to emit a E_USER_DEPRECATED error when calling a deprecated function, combined with either parsing a doc comment for the /** @deprecated */ 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. ReflectionFunctionAbstract::isDeprecated() always returns false for userland functions - and doc comments / custom attributes are unavailable for internal functions. For class constants the deprecation behavior cannot be emulated at all.

A new attribute #[\Deprecated] shall be added:

#[Attribute(Attribute::TARGET_METHOD|Attribute::TARGET_FUNCTION|Attribute::TARGET_CLASS_CONSTANT)]
final class Deprecated
{
    public readonly ?string $message;
 
    public function __construct(?string $message = null) {}
}

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

The $message 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.

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

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

<?php
 
#[\Deprecated]
function test() {
}
 
#[\Deprecated("use test() instead")]
function test2() {
}
 
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
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
 
#[\Deprecated]
function test() {
}
 
$r = new ReflectionFunction('test');
 
var_dump($r->isDeprecated()); // bool(true)
 
?>

<?php
 
class Clazz {
    #[\Deprecated]
    public const OLD_WAY = 'foo';
}
 
$r = new ReflectionClassConstant(Clazz::class, 'OLD_WAY');
 
var_dump($r->isDeprecated()); // bool(true)
 
?>

<?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"
}
*/
 
?>

Further examples are given by the newly added tests within the PR for this RFC.

Deprecated 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.

Next minor (PHP 8.4).

None.

The #[\Deprecated] 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 /** @deprecated */ 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.

None.

None.

None.

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

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

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

n/a

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

• Changes to the runtime behavior of deprecated functions and class constants are out of scope of this RFC (i.e. not emitting the E_DEPRECATED error for internal functions).
• Making the Deprecated 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.