Question

There are some cases when you override a method which has type hinted input parameter like this:

class FooParent
{
    public function bar(BazInterface $baz)
    {
        // ...
    }
}

And you want to allow passing null values as input parameters.

If you remove interface type hint

class Foo extends FooParent
{
    public function bar($baz)
    {
        // ...
    }
}

you’ll get an error like this:

Fatal error: Declaration of Foo::bar() must be compatible with that of FooParent::bar()

How can you allow null values without changing the parent class?

This is a real world example since parent class can be part of the third party library or framework, so changing it is not an option.

Answer 1

Solution is to add default null value to input parameter like this:

class Foo extends FooParent
{
    public function bar(BazInterface $baz = null)
    {
        // ...
    }
}

This is not what I expected since default value assigns default value to a variable if not provided, I didn’t expect it to affect allowed input. But I saw example on http://php.net/manual/en/language.oop5.typehinting.php, so I decided to document it here. Hope someone will find it useful.