What defines an attribute class in PHP 8?

Question

I am using PHP 8 attributes, the documentation of this feature is still rudimentary, it only contains code examples and the api is not documented so I am experiencing the use of the resource.

class MeuAtributo {}

#[MeuAtributo]
class Teste {}

$reflection = new ReflectionClass(Teste::class);
$attributes = $reflection->getAttributes();
var_dump($attributes[0]->newInstance());

When executing this code the following error occurs in the call of newInstance():

Attempting to use non-attribute class "MeuAtributo" as attribute in /code/index.php:10

• What is the attribute class mentioned in the message?
• How to correct this error?

Answer 1

TD; DR:

To define an add class attribute class #[Attribute] before class:

namespace App;

use Attribute;

#[Attribute]
class MeuAtributo {}

The use is necessary only when the class uses namespace, as in this example.

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Understanding class of attributes

The definition of attribute classes is described on this page of the manual (no translation). Still I find it useful to make a parallel with languages that already use this resource, such as C# for example, because it lacks conceptual description of the resource.

Attributes provide an efficient method of associating metadata, or declarative information, to code (classes, methods and properties, functions, class constants, parameters). After an attribute is associated with a program entity, the attribute can be consulted at runtime using a technique called reflection.

Attributes have the following properties:

• Attributes add metadata to your program. Metadata is information about the types defined in a program. You can add custom attributes to specify any additional information required.
• You can apply one or more attributes to classes, methods and properties, etc.
• Attributes can accept arguments.
• Your program can examine its own metadata or metadata in other programs using reflection.

An attribute class is a class that derives directly or indirectly from the class Attribute, which makes the identification of attribute definitions in the metadata quick and easy. Suppose you want to mark types with the name of the programmer who wrote the type. You can define a class of attributes Author customized.

In practice

Attribute has an optional parameter to limit where this attribute class can be used, this argument accepts a combination of constants using | (bit operator "OR"):

Attribute::TARGET_ALL;
Attribute::TARGET_CLASS;
Attribute::TARGET_CLASS_CONSTANT;
Attribute::TARGET_FUNCTION;
Attribute::TARGET_METHOD;
Attribute::TARGET_PARAMETER;
Attribute::TARGET_PROPERTY;

For example:

#[Attribute(Attribute::TARGET_METHOD | Attribute::TARGET_FUNCTION)]
class Exemplo {}

Will allow the attribute class Exemplo is used only as method and function attributes.

About the mistakes

The error highlighted is actually an exception and can be captured with try/catch. It occurs when there is a problem with the implementation, whether it is the missing attribute (the error pointed out in the question) or a use restriction (described above).