PHP MySQL login function -

i'm using this:

function authuser($username, $password){     connectdb();     $sql = "select id, username users username = '".$username."' , password = '".$password."'";     $result = mysql_query($sql);     $num_rows = mysql_num_rows($result);     if ($num_rows > 0){         while ($row = mysql_fetch_array($result)){             $username = $row['username'];             session_start();              session_register('username');         return $username;          }     }     closeconn(); } 

with combination of this:

$auth = authuser($username, $password); if (isset($username)){     header( "location: index.php" ); } 

and on index.php (where redirect them if successful login) i'm trying echo $username. nothing showing? ideas? function problem?

edit:

have changed so:

if ($num_rows > 0){     while ($row = mysql_fetch_array($result)){         $_session['username'] = $row['username'];       return true;      } } 

is right?

i change:

while ($row = mysql_fetch_array($result)){     $_session['username'] = $row['username'];   return true;  } 

into:

$row = mysql_fetch_array($result); $_session['username'] = $row['username']; 

because want login , 1 person out