if(!empty($username)&& !empty($pass)){
    $sql = "SELECT id_adm FROM admin WHERE username='$username'AND pass='$pass'";
    if($query = mysql_query($sql)){
        $qnr = mysql_num_rows($query);
        if($qnr === 0){ ?>
           <div class='alert'>Name or Password is incorrect .<span class='close'>X</span></div>
    <?php }else if($qnr === 1){
        $id = mysql_result($query, 0, 'id_adm');
        $_SESSION['id']=$id;
        header('Location: index.php');
        }
    }
}

if(!empty($username)&& !empty($pass)){
    $sql = "SELECT id_adm FROM admin WHERE username='$username'AND pass='$pass'";
    if($query = mysqli_query($con,$sql)){
        $qnr = mysqli_num_rows($query);
        if($qnr === 0){ ?>
            <div class='alert'>Name or Password is incorrect.<span class='close'>X</span></div>
        <?php }else if($qnr === 1){
            $row= mysqli_fetch_assoc($con,$query);
            $id = $row['id_adm'];
            $_SESSION['id']=$id;
            header('Location: index.php');
        }
    }
}