====== PHP RFC: Dynamic class constant fetch ======

  * Date: 2022-11-04
  * Author: Ilija Tovilo, tovilo.ilija@gmail.com
  * Status: Implemented
  * Target Version: PHP 8.3
  * Implementation: https://github.com/php/php-src/pull/9793

===== Proposal =====

PHP implements various ways of looking up members name.

  - Variables <php>$$foo</php>
  - Properties <php>$foo->$bar</php>
  - Static properties <php>Foo::${$bar}</php>
  - Methods <php>$foo->{$bar}()</php>
  - Static methods <php>Foo::{$bar}()</php>
  - Classes for static properties <php>$foo::$bar</php>
  - Classes for static methods <php>$foo::bar()</php>

One notable exception are class constants.

<code php>
class Foo {
    const BAR = 'bar';
}
$bar = 'BAR';

// This is currently a syntax error
echo Foo::{$bar}; 

// Instead, the `constant` function must be used
echo constant(Foo::class . '::' . $bar);
</code>

This limitation seems rather arbitrary. This RFC proposes to introduce the syntax described above.

===== Semantics =====

==== Non-existent class constants ====

Trying to access a non-existent class constant by name throws a <php>Error</php>. This behavior is equivalent to a normal
class constant fetch.

<code php>
class Foo {}

$bar = 'BAR';
echo Foo::{$bar};
// Error: Undefined constant Foo::BAR
</code>

==== {} expression type ====

The result of the expression in the braces <php>{}</php> must be of type <php>string</php>. If it is not, a
<php>TypeError</php> is thrown.

<code php>
echo Foo::{[]};
// TypeError: Cannot use value of type array as class constant name
</code>

==== Order of execution ====

Unfortunately, the order of execution for member lookups is inconsistent.

<code php>
function test($value) {
    echo $value . "\n";
    return $value;
}

class Foo implements ArrayAccess {
    public function __get($property) {
        echo 'Property ' . $property . "\n";
        return $this;
    }
    
    public function __call($method, $arguments) {
        echo 'Method ' . $method . "\n";
        return $this;
    }
    
    public static function __callStatic($method, $arguments) {
        echo 'Static method ' . $method . "\n";
        return static::class;
    }

    public function offsetGet($offset): mixed {
        echo 'Offset ' . $offset . "\n";
        return $this;
    }
    public function offsetExists($offset): bool {}
    public function offsetSet($offset, $value): void {}
    public function offsetUnset($offset): void {}
}

$foo = new Foo();

$foo->{test('foo')}->{test('bar')};
// foo
// bar
// Property foo
// Property bar

$foo->{test('foo')}()->{test('bar')}();
// foo
// Method foo
// bar
// Method bar

Foo::{test('foo')}()::{test('bar')}();
// foo
// Static method foo
// bar
// Static method bar

$foo[test('foo')][test('bar')];
// foo
// bar
// Offset foo
// Offset bar

// Can't be demonstrated because there is no __getStatic
Foo::${test('foo')}::${test('bar')};
// foo
// Static property foo
// bar
// Static property bar
</code>

Property and array accesses evaluate all expressions in the chain before performing any of the actual operations. The reason for this is rather [[https://www.npopov.com/2017/04/14/PHP-7-Virtual-machine.html#writes-and-memory-safety|technical]]. Basically, no userland code must run between property or array accesses to avoid reallocation and thus potentially invalidating pointers. This problem does not apply to class constants. Thus, the simpler and more intuitive in-order approach is chosen. Evaluation order for class constants is also unlikely to matter since chaining them is of dubious usefulness.

<code php>
Foo::{test('foo')}::{test('bar')};
// foo
// Class constant foo
// bar
// Class constant bar
</code>

==== Magic 'class' constant ====

For completeness, accessing the magic <php>class</php> constant dynamically is allowed.

<code php>
namespace Foo;

$class = 'class';
echo Bar::{$class};
// Foo\Bar
</code>

==== Enums ====

The feature works for enum cases as expected.

===== Future scope =====

==== Interaction with ?? ====

This RFC proposes no change in the interaction between class constant fetches and the null-coalescing operator
<php>??</php>. That is, <php>Foo::{$bar} ?? null;</php> will throw an <php>Error</php> if the given constant does not 
exist. It would be possible to suppress this error as is done for other types of member accesses. However, it's not
clear whether this is desirable, especially for explicit class constant fetches. This change can be made in the future
with no backwards compatibility break.

===== Vote =====

Voting starts 2022-12-22 and ends 2023-01-05. 

As this is a language change, a 2/3 majority is required.

<doodle title="Add dynamic class constant fetches to PHP 8.3?" auth="ilutov" voteType="single" closed="true">
   * Yes
   * No
</doodle>