registration help

[mancombe]

hi guys i have made a registration page and login that works the problem is people can register the same username, here is what i have tried so far to prevent this but not having much luck.

 

if($num_rows==0)
{

  $sql="INSERT INTO $tbl_name VALUES('$id','$Username','$Password','$firstName','$lastName','$email')";

  $res=mysql_query($sql) or die("cannot execute");

  echo "<h1> You are Successfully Registered</h1>"; 
  }

else
    {
      echo " <h1>That user already Exist!</h1>" ;

     }  

?>    

 

 

any advice would be good 

[Daniel0]

How doesn't it work. You might want to show the code that comes before this as well.