Attribute syntax

There are several parts to the attributes syntax. First, an attribute declaration is always enclosed with a starting #[ and a corresponding ending ]. Inside, one or many attributes are listed, separated by comma. The attribute name is an unqualified, qualified or fully-qualified name as described in Using Namespaces Basics. Arguments to the attribute are optional, but are enclosed in the usual parenthesis (). Arguments to attributes can only be literal values or constant expressions. Both positional and named arguments syntax can be used.

Attribute names and their arguments are resolved to a class and the arguments are passed to its constructor, when an instance of the attribute is requested through the Reflection API. As such a class should be introduced for each attribute.

<?php
// a.php
namespace MyExample;

use Attribute;

#[Attribute]
class MyAttribute
{
    const VALUE = 'value';

    private $value;

    public function __construct($value = null)
    {
        $this->value = $value;
    }
}

// b.php

namespace Another;

use MyExample\MyAttribute;

#[MyAttribute]
#[\MyExample\MyAttribute]
#[MyAttribute(1234)]
#[MyAttribute(value: 1234)]
#[MyAttribute(MyAttribute::VALUE)]
#[MyAttribute(array("key" => "value"))]
#[MyAttribute(100 + 200)]
class Thing
{
}

#[MyAttribute(1234), MyAttribute(5678)]
class AnotherThing
{
}