Question

I’m wondering if there is any good reason why this behaviour is possible in the current PHP 5.4 implementation:

trait T {
    public function test(PDO $pdo) {}
}

class C {
    use T;
    public function test(DOMDocument $dom) {}
}

I thought that the fact that a class uses a trait, guaranteed that this class had a specific interface available. But here, if we inadvertently override the trait method for another purpose, we don’t even receive a Strict Standards notice, as with classic inheritance.

Is this specifically allowed on purpose? What for?

Answer 1

This behavior is documented. From php.net (http://php.net/manual/en/language.oop5.traits.php):

An inherited member from a base class is overridden by a member
inserted by a Trait. The precedence order is that members from the
current class override Trait methods, which in return override
inherited methods.

No reason for notices here.

Edit:

I took a look on some more serious literature to shed some light on this topic 🙂 . Looks like that such behavior is a part of traits’ definition. They are ment to work this way. This is from research "Traits: Composable Units of Behavior"(Proceedings of the European Conference on Object-Oriented Programming):

Another property of trait composition is that the composition order is
irrelevant, and hence conflicting trait methods must be explicitly
disambiguated (cf. section 3.5). Conflicts between methods defined in
classes and methods defined by incorporated traits are resolved using
the following two precedence rules.

– Class methods take precedence over trait methods.

– Trait methods take precedence over superclass methods. This follows from the flattening property, which states that trait methods behave as if they were defined in the class itself.

You can read more here: http://scg.unibe.ch/archive/papers/Scha03aTraits.pdf