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Showing content from https://docs.hhvm.com/hack/attributes/attribute-specification below:

Attributes: Introduction

Attributes attach metadata to Hack definitions.

Hack provides built-in attributes that can change runtime or typechecker behavior.

<<__Memoize>>
function add_one(int $x): int {
  return $x + 1;
}

You can attach multiple attributes to a definition, and attributes can have arguments.

<<__ConsistentConstruct>>
class OtherClass {
  <<__Memoize, __Deprecated("Use FooClass methods instead")>>
  public function addOne(int $x): int {
    return $x + 1;
  }
}

You can define your own attribute by implementing an attribute interface in the HH namespace.

class Contributors implements HH\ClassAttribute {
  public function __construct(private string $author, private ?keyset<string> $maintainers = null) {}
  public function getAuthor(): string {
    return $this->author;
  }
  public function getMaintainers(): keyset<string> {
    return $this->maintainers ?? keyset[$this->author];
  }
}

<<Contributors("John Doe", keyset["ORM Team", "Core Library Team"])>>
class MyClass {}

<<Contributors("You")>>
class YourClass {}

Other common attribute interfaces are HH\FunctionAttribute, HH\MethodAttribute and HH\PropertyAttribute, but see the Hack interface reference for the full list.

You need to use reflection to access attribute arguments.

Given the MyClass example defined above:

$rc = new ReflectionClass('MyClass');
$my_class_contributors = $rc->getAttributeClass(Contributors::class);
$my_class_contributors?->getAuthor(); // "John Doe"
$my_class_contributors?->getMaintainers(); // keyset["ORM Team", "Core Library Team"]

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