Javel's notes

function test(int $a, bool $b) {}

test(1, false) // ok
test(1)        // ArgumentCountError
test(1, null)  // TypeError Argument #2 ($b) must be of type bool, null given

a & b are both required.

function test(int $a, ?bool $b) {}

test(1, false) // ok
test(1)        // ArgumentCountError
test(1, null)  // ok

b is nullable but always required.

function test(int $a, bool $b = false) {}

test(1, false) // ok
test(1)        // ok, $b = false
test(1, null)  // TypeError Argument #2 ($b) must be of type bool, null given

b is optional, but not nullable (default value will be false)

function test(int $a, ?bool $b = false) {}

test(1, false) // ok
test(1)        // ok, $b = false
test(1, null)  // ok

b is optional and nullable. When not specified on function call, b will be false by default. It's value may also be null.