Probably call with something more like:
$r = $api->editCategoryPermissions(641, array(
array('group_name' => 'admins', 'permission_type' => 1)
));
Probably call with something more like:
$r = $api->editCategoryPermissions(641, array(
array('group_name' => 'admins', 'permission_type' => 1)
));