Why does the page callback have two arguments?

Question

function menufun_menu() {
  $items['menufun/%/bar/baz'] = array(
    'title' => 'Hi',
    'page callback' => 'menufun_hello',
    'page arguments' => array(1), // The matched wildcard.
    'access callback' => TRUE,
  );

  return $items;
}

/**
* Page callback.
*/
function menufun_hello($a = NULL, $b = NULL) {
  return t('Hello. $a is @a and $b is @b', array('@a' => $a, '@b' => $b));
}

In the menufun_menu() function, the page argument for the menufun_hello callback is only one. Why does menufun_hello() have two parameters?

Answer 1

Menu callbacks are passed first the arguments defined in page arguments then excess arguments from the url.

So in the above example:

menufun/1/bar/baz

Will provide these vars:

$a = 1;
$b = NULL;

If you however provide an extra argument in the url:

menufun/1/bar/baz/2

You will transfer it to the menu callback:

$a = 1;
$b = 2;

The other advantedge is like tim writes, that it allows reuse of the same code for various cases, making the module more maintainable.

Answer 2

In this case, menufun_hello() takes up to two arguments, but as little as zero, since it provides defaults. This means that menufun_menu() could later pass another argument, and the function signature wouldn't have to change.