Amfphp service call failure

1

When I call a service function like this

 function addCat() {
  return mysql_query (
   "INSERT INTO categories
   VALUES (NULL, link, icon, caption, description, sound)"
  );
 }

I get true, which is fine, but when I try to pass some params i get false

 function addCategory($link, $icon, $caption, $description, $sound) {
  return mysql_query (
   "INSERT INTO categories
   VALUES (NULL, $link, $icon, $caption, $description, $sound)"
  );
 }

Somebody knows what can cause that problem?