Why so complicated

You could just use associative arrays, as its already a common usage (like in the zend fw).

function myFunc(array $params= array()) {
if(isset($params['arg1'])) doStuff();
}

This needs no extra syntax and extra classes.
So, why so complicated?

Krnl

Oh i forgot. The method call may look like this:
MyFunc(array('arg1' => 'foo', 'ignored' => 'bar'));

a different approach to the same issue.
With your solution, the function would need to be built that way.
This package is just a fancier way to do that.
Argueably, using arrays for the class calling would be faster.

Not only the call would be faster, you can use my mentioned example in another class. Yours not. Zend did this on purpose, otherwise they would have found a similar solution like yours. You might also extend functions with array parameters:

cass Foo
{
function foo(array $params = array())
{
if(isset($params['foo']) $this->doStuff();
}
protected function doStuff()
{
echo 'stuff'.PHP_EOL;
}
}
class Bar
{
function foo(array $params = array())
{
parent::foo($params);
if(isset($params['bar']) $this->doMoreStuff();
}
protected function doMoreStuff()
{
echo 'more stuff'.PHP_EOL;
}
}

$x = new Bar();
$x->foo(array('foo' => 'john', 'bar' => 'doe'));

//Output:
/*
stuff
more stuff
*/

class Bar (of course) extends Foo ;)