PHP implements various ways of looking up members name.
$$foo$foo->$barFoo::${$bar}$foo->{$bar}()Foo::{$bar}()$foo::$bar$foo::bar()One notable exception are class constants.
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);
This limitation seems rather arbitrary. This RFC proposes to introduce the syntax described above.
Trying to access a non-existent class constant by name throws a Error. This behavior is equivalent to a normal
class constant fetch.
class Foo {} $bar = 'BAR'; echo Foo::{$bar}; // Error: Undefined constant Foo::BAR
The result of the expression in the braces {} must be of type string. If it is not, a
TypeError is thrown.
echo Foo::{[]}; // TypeError: Cannot use value of type array as class constant name
Unfortunately, the order of execution for member lookups is inconsistent.
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
Property and array accesses evaluate all expressions in the chain before performing any of the actual operations. The reason for this is rather 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.
Foo::{test('foo')}::{test('bar')}; // foo // Class constant foo // bar // Class constant bar
For completeness, accessing the magic class constant dynamically is allowed.
namespace Foo; $class = 'class'; echo Bar::{$class}; // Foo\Bar
The feature works for enum cases as expected.
This RFC proposes no change in the interaction between class constant fetches and the null-coalescing operator
??. That is, Foo::{$bar} ?? null; will throw an Error 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.
Voting starts 2022-12-22 and ends 2023-01-05.
As this is a language change, a 2/3 majority is required.