php user varification

[msz]

hy to every one..
iam going to filter user input data using 

1. if(!filter_input(INPUT_POST,'$username')||!filter_input(INPUT_POST,'$password'))
2. {
3. echo "username or password already exists....";
4.  
5. }
6.  
7. else
8. {
9. $sql = "INSERT INTO users (Username,Password) VALUES (:username,:password)";
10. $q=$dbh->prepare($sql);
11. $q->execute(array(':username'=>$username,':password'=>$password));
12. $count=$q->rowCount();
13. echo "inserted $count rows.\n";

but it show an error...
the error is that it simply show the message

1. username or password already exists

if the username or password is not exists it show the same message?

if there is another metod in pdo to filter user data then plz proived the idea.