Attributes overview

    (PHP 8)

    PHP attributes provide structured, machine-readable metadata for classes, methods, functions, parameters, properties, and constants. They can be inspected at runtime via the Reflection API, enabling dynamic behavior without modifying code. Attributes provide a declarative way to annotate code with metadata.

    Attributes enable the decoupling of a feature's implementation from its usage. While interfaces define structure by enforcing methods, attributes provide metadata across multiple elements, including methods, functions, properties, and constants. Unlike interfaces, which enforce method implementations, attributes annotate code without altering its structure.

    Attributes can complement or replace optional interface methods by providing metadata instead of enforced structure. Consider an ActionHandler interface that represents an operation in an application. Some implementations may require a setup step while others do not. Instead of forcing all classes implementing ActionHandler to define a setUp() method, an attribute can indicate setup requirements. This approach increases flexibility, allowing attributes to be applied multiple times when necessary.

    Example #1 Implementing optional methods of an interface with Attributes

    <?php
    interface ActionHandler
    {
    public function     execute();
    }

    #[    Attribute]
    class     SetUp {}

    class     CopyFile implements ActionHandler
    {
    public     string $fileName;
    public     string $targetDirectory;

    #[    SetUp]
    public function     fileExists()
    {
    if (!    file_exists($this->fileName)) {
    throw new     RuntimeException("File does not exist");
    }
    }

    #[    SetUp]
    public function     targetDirectoryExists()
    {
    if (!    file_exists($this->targetDirectory)) {
    mkdir($this->targetDirectory);
    } elseif (!    is_dir($this->targetDirectory)) {
    throw new     RuntimeException("Target directory $this->targetDirectory is not a directory");
    }
    }

    public function     execute()
    {
    copy($this->fileName, $this->targetDirectory . '/' . basename($this->fileName));
    }
    }

    function     executeAction(ActionHandler $actionHandler)
    {
    $reflection = new ReflectionObject($actionHandler);

    foreach (    $reflection->getMethods() as $method) {
    $attributes = $method->getAttributes(SetUp::class);

    if (    count($attributes) > 0) {
    $methodName = $method->getName();

    $actionHandler->$methodName();
    }
    }

    $actionHandler->execute();
    }

    $copyAction = new CopyFile();
    $copyAction->fileName = "/tmp/foo.jpg";
    $copyAction->targetDirectory = "/home/user";

    executeAction($copyAction);

    Found A Problem?

    Learn How To Improve This Page • Submit a Pull Request • Report a Bug
    ＋add a note

    User Contributed Notes 2 notes

    up
    42
    Harshdeep
    3 years ago
    While the example displays us what we can accomplish with attributes, it should be kept in mind that the main idea behind attributes is to attach static metadata to code (methods, properties, etc.).

    This metadata often includes concepts such as "markers" and "configuration". For example, you can write a serializer using reflection that only serializes marked properties (with optional configuration, such as field name in serialized file). This is reminiscent of serializers written for C# applications.

    That said, full reflection and attributes go hand in hand. If your use case is satisfied by inheritance or interfaces, prefer that. The most common use case for attributes is when you have no prior information about the provided object/class.

    <?php
    interface JsonSerializable
    {
    public function     toJson() : array;
    }
    ?>

    versus, using attributes,
    <?php

    #[Attribute]
    class     JsonSerialize
    {
    public function     __constructor(public ?string $fieldName = null) {}
    }

    class     VersionedObject
    {
    #[    JsonSerialize]
    public const     version = '0.0.1';
    }

    public class     UserLandClass extends VersionedObject
    {
    #[    JsonSerialize('call it Jackson')]
    public     string $myValue;
    }

    ?>
    The example above is a little extra convoluted with the existence of the VersionedObject class as I wished to display that with attribute mark ups, you do not need to care how the base class manages its attributes (no call to parent in overriden method).
    up
    37
    Florian Krmer
    2 years ago
    I've tried Harshdeeps example and it didn't run out of the box and I think it is not complete, so I wrote a complete and working naive example regarding attribute based serialization.

    <?php
    declare(strict_types=1);

    #[    Attribute(Attribute::TARGET_CLASS_CONSTANT|Attribute::TARGET_PROPERTY)]
    class     JsonSerialize
    {
    public function     __construct(public ?string $fieldName = null) {}
    }

    class     VersionedObject
    {
    #[    JsonSerialize]
    public const     version = '0.0.1';
    }

    class     UserLandClass extends VersionedObject
    {
    protected     string $notSerialized = 'nope';

    #[    JsonSerialize('foobar')]
    public     string $myValue = '';

    #[    JsonSerialize('companyName')]
    public     string $company = '';

    #[    JsonSerialize('userLandClass')]
    protected ?    UserLandClass $test;

    public function     __construct(?UserLandClass $userLandClass = null)
    {
    $this->test = $userLandClass;
    }
    }

    class     AttributeBasedJsonSerializer {

    protected const     ATTRIBUTE_NAME = 'JsonSerialize';

    public function     serialize($object)
    {
    $data = $this->extract($object);

    return     json_encode($data, JSON_THROW_ON_ERROR);
    }

    protected function     reflectProperties(array $data, ReflectionClass $reflectionClass, object $object)
    {
    $reflectionProperties = $reflectionClass->getProperties();
    foreach (    $reflectionProperties as $reflectionProperty) {
    $attributes = $reflectionProperty->getAttributes(static::ATTRIBUTE_NAME);
    foreach (    $attributes as $attribute) {
    $instance = $attribute->newInstance();
    $name = $instance->fieldName ?? $reflectionProperty->getName();
    $value = $reflectionProperty->getValue($object);
    if (    is_object($value)) {
    $value = $this->extract($value);
    }
    $data[$name] = $value;
    }
    }

    return     $data;
    }

    protected function     reflectConstants(array $data, ReflectionClass $reflectionClass)
    {
    $reflectionConstants = $reflectionClass->getReflectionConstants();
    foreach (    $reflectionConstants as $reflectionConstant) {
    $attributes = $reflectionConstant->getAttributes(static::ATTRIBUTE_NAME);
    foreach (    $attributes as $attribute) {
    $instance = $attribute->newInstance();
    $name = $instance->fieldName ?? $reflectionConstant->getName();
    $value = $reflectionConstant->getValue();
    if (    is_object($value)) {
    $value = $this->extract($value);
    }
    $data[$name] = $value;
    }
    }

    return     $data;
    }

    protected function     extract(object $object)
    {
    $data = [];
    $reflectionClass = new ReflectionClass($object);
    $data = $this->reflectProperties($data, $reflectionClass, $object);
    $data = $this->reflectConstants($data, $reflectionClass);

    return     $data;
    }
    }

    $userLandClass = new UserLandClass();
    $userLandClass->company = 'some company name';
    $userLandClass->myValue = 'my value';

    $userLandClass2 = new UserLandClass($userLandClass);
    $userLandClass2->company = 'second';
    $userLandClass2->myValue = 'my second value';

    $serializer = new AttributeBasedJsonSerializer();
    $json = $serializer->serialize($userLandClass2);

    var_dump(json_decode($json, true));
    ＋add a note
    To Top