How to prevent duplicate Usernames in php/mysql

Question

(This is my first try with php)

I have a very basic register page. http://graves-incorporated.com/test_sites/member_test/register/register.php

* I j

Answer 1

Please fill out all fields.';

    }elseif($_POST['password'] != $_POST['conf_pass']){
        echo 'Your Passwords do not match.';
    }else{

        $dup = mysql_query("SELECT username FROM users WHERE username='".$_POST['username']."'");
        if(mysql_num_rows($dup) >0){
            echo 'username Already Used.';
        }
        else{
            $url = 'http://graves-incorporated.com/test_sites/plantation_park_2012/';
            echo '';

            $sql = mysql_query("INSERT INTO users VALUES(NULL, '$_POST[username]', '$_POST[password]', '$_POST[email]')");     
            if($sql){
                 echo 'Congrats, You are now Registered.';
            }
            else{
                echo 'Error Registeration.';
            }
        }
    }
}
?>