hook_pathauto_clean_alias doesn't do anything

Hi,

I tried to make an implementation of the hook pathauto_clean_alias in my module, unfortunately I was unable to find any documentation related to this hook. So, I took a look at the code and found the place where it's called (line 264 in pathauto.inc).

module_invoke_all('pathauto_clean_alias', $output);

This suggests that $output is passed as reference and my implementation should therefore look as follows.

function mymodule_pathauto_clean_alias(&$alias) {
  if(something) $alias = 'something else';
}

But now I get the following error message (PHP 5.3):

warning: Parameter 1 to virtual_sites_pathauto_clean_alias() expected to be a reference, value given in /bla/includes/module.inc on line 483.

So, passing a string as reference is somehow not allowed... Obviously the following code does not work either.

function mymodule_pathauto_clean_alias($alias) {
  if(something) return 'something else';
  else return $alias;
}

This seems way cleaner, so I've changed the call of hook_pathauto_clean_alias (at line 264 in pathauto.inc) as follows.

foreach (module_implements('pathauto_clean_alias') as $name) {
    $function = $name .'_pathauto_clean_alias';
    $output = $function($output);
}

Since the hook is undocumented I have no clue whether my implementation should modify the alias as a reference or should return the modified alias. Hence I have no idea whether this is an actual bug, something related to the changed regarding references in PHP5.3 or my own stupidity.