Contravariance¶
Contravariance allows a parameter type to be less specific in a child method, than that of its parent.
This means that a parameter $param` in the method foo of the child class X can be a parent of the type in the method foo of the parent class.
Less specific types can be : the same type W, a parent class of the W type (here V), an interface implemented by the type W, a union type, such as W|string. In particular, the type may become nullable.
It cannot be a totally different type, an intersectional type, such as W&A, a different class that implements the same interfac than W, or a different scalar type.
<?php
class V {}
class W extends V {
// This method accepts an object compatible with w, but less specific
function foo(W $param) { }
}
class X extends W {
function foo(V $param) { }
}
?>
See also https://dev.to/ashallendesign/covariance-and-contravariance-in-php-3cim
Related : Covariance, Nullable, Intersection Type, Union Type, Scalar Types
Added in PHP 7.4+
