成功登录php后需要在字段中输入用户名

Question

我在成功登录后面临使用用户名的问题。

以下是登录代码：

<?php
//validation error
if(isset($_REQUEST['validationError']))
{
    if($_REQUEST['validationError']==1)
    {
        $username='Username required';
    }
    if($_REQUEST['validationError']==2)
    {
        $password='Password required';
    }
    if($_REQUEST['validationError']==3)
    {
        $not_match='User not exist. Please give correct information';
    }
}
?>
    <p><span style="color:red;display:<?php echo isset($not_match)?'block':'none' ?>"><?php echo isset($not_match)?$not_match:''; ?><span></p>

    <p><input size="40px" type="text" name="username" value="<?php echo isset($_POST['username'])?$_POST['username']:''; ?>" value="" placeholder="Username"></p>

    <p><span style="color:red;display:<?php echo isset($username)?'block':'none' ?>"><?php echo isset($username)?$username:''; ?><span></p>

    <p><input size="40px" type="password" name="password" value="" placeholder="Password"></p>
    <p><span style="color:red;display:<?php echo isset($password)?'block':'none' ?>"><?php echo isset($password)?$password:''; ?><span></p>

    <p class="submit"><input type="submit" name="submit" value="Login"></p>
 </form>

以下是登录检查的代码：

if($username !='' && $password !='')
{
    include('database_connection.php');
    $result = mysql_query("SELECT * from user WHERE username='$username' AND password='$password'");

    if(mysql_num_rows($result)==0)
    {
        $validationError=3;  
    }
}


if(isset($validationError) && $validationError==0)
{
    //session set
    session_start();
    $_SESSION['isLogin']=1;
    //redirect to homepage with successfull message
    header("Location:$base_url/index.php?validationError=$validationError");
}
else
{
    //redirect to login_form with msg
    header("Location:$base_url/login_form.php?validationError=$validationError");
}

现在我想在下拉框字段中获取用户名值。

下面是代码          

$sql = "SELECT username FROM `user`";
$res = mysql_query($sql);
while ($row1 = mysql_fetch_array($res)) {
    $selected = ($val == $row1['username'] ? 'selected="selected"' : '');
    echo '<option value ="' . $row1['username'] . '" '. $selected .'>' . $row1['username'] . '</option>';

}
?>

但我在这个领域没有任何价值。