如何从drupal 7中的URL获取页面参数

I have created on menu hook in drupal 7

$items['assistance/product-help/%'] = array(
       'page callback' => 'product_water_serach',
   'access arguments' => array('access content'),
       'page arguments' => array(2),
       'access callback' => TRUE,
);

My my url is

http://example.com/assistance/product-help/abcd+%20test

my function is

function product_water_serach($product_name)
   {
     echo $product_name; 
   }

output is : abcd test

My requirement is output should be : abcd+ test

Kindly suggest solution for above.