Mixin¶
A mixin is a class-level construct that provides a set of methods intended to be incorporated into other classes, without forming a base-class relationship. Unlike inheritance, a mixin does not impose an is-a relationship. Unlike an interface, it supplies concrete implementations. Unlike a trait (as PHP defines it), a mixin is itself an instantiable class or a first-class language entity that is composed through language syntax rather than a use directive inside the class body.
Languages such as Dart, Kotlin (via delegation), and Scala (via self-types and stackable modifications) offer mixins as distinct, class-level features.
PHP does not have mixins. PHP traits are the closest analogue: they provide method reuse without inheritance. However, traits differ from true mixins in several ways:
A trait is not a class and cannot be instantiated on its own.
A trait is composed inside the class definition (it cannot be applied after the fact).
A trait cannot extend another class; a mixin in some languages can carry its own class hierarchy.
PHP traits are resolved at compile time; some mixin systems allow run-time composition.
PHP interfaces with default implementations (as in Java) are also a common comparison point, but PHP interfaces carry no implementation at all.
<?php
// PHP uses traits as the closest equivalent to mixins.
// They share the intent but differ from class-level mixins.
trait Timestampable {
private \DateTimeImmutable $createdAt;
public function getCreatedAt(): \DateTimeImmutable {
return $this->createdAt;
}
}
class Article {
use Timestampable; // composed here, not applied externally
}
// A true class-level mixin (as in Dart) would be declared separately
// and applied without modifying the target class source.
?>
See also Dart mixins and Mixins vs Traits.
Related : Trait, Use In Traits, Open Class, Polymorphism, Method Resolution Order (MRO)
