Any reason why the following has never been implemented for optional function arguments?

function test($a, $b = 1, $c = 2)
{
//do something
}

test("hello",,7);

What I would like the test function to see would be
$a = "hello"
$b = 1
$c = 7

Since I skipped the second argument, I would like the default value to kick in. This would be really useful!