php Login script issue

Hi,

Its just a login and password validation that I am trying to achieve. If the username is correct then the person is able to view certain page, if incorrect then he is directed elsewhere.

<?
$userid=mysql_real_escape_string($userid);
$password=mysql_real_escape_string($password);

if($rec=mysql_fetch_array(mysql_query("SELECT * FROM tablename WHERE userName='$userName' AND password = '$password'"))){
        if(($rec['userName']==$userName)&&($rec['password']==$password)){
         include "../include/newsession.php";
            echo "<p class=data> <center>Successfully,Logged in<br><br> Log OUT <br><br>Click here if your browser is not redirecting automatically or you don't want to wait.<br></center>";
     print "<script>";
       print " self.location='submit-store-details.php';"; // Comment this line if you don't want to redirect
          print "</script>";

                                }
                }
        else {

                session_unset();
echo "Wrong Login. Use your correct  Userid and Password and Try <br><center><input type='button' value='Retry' onClick='history.go(-1)'></center>";
               
        }
?>

I am getting this error when I am using this code:

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in thispage.php on line 37
Wrong Login. Use your correct Userid and Password and Try

Why does it show up everytime and whats wrong with mysql_fetch_array().

Please advice also if there is some other way available please help me try that.

Thanks,

Chris