$a = null) {} function noDefaultValue(Countable $a) {} // PHP 7.1+: using a nullable type. function foo(?Countable $a = null) {} // PHP 8.0+: using a union type with null // or the explicit mixed type. function bar(mixed $a = null) {} function baz(Countable|null $a = null) {} ]]> Countable $a = null) {} ]]>