用户无法判断已存在

<?php
include_once 'captcha/securimage.php';
$securimage = new Securimage();
require 'includes/db.php';
require 'includes/init.php';?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><!-- Apple iOS and Android stuff (do not remove) -->
<meta name="apple-mobile-web-app-capable" content="no" />
<meta name="apple-mobile-web-app-status-bar-style" content="black" /><meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=no,maximum-scale=1" /><!-- Required Stylesheets -->
<link rel="stylesheet" type="text/css" href="css/reset.css" media="screen" />
<link rel="stylesheet" type="text/css" href="css/text.css" media="screen" />
<link rel="stylesheet" type="text/css" href="css/fonts/ptsans/stylesheet.css" media="screen" /><link rel="stylesheet" type="text/css" href="css/core/form.css" media="screen" />
<link rel="stylesheet" type="text/css" href="css/core/login.css" media="screen" />
<link rel="stylesheet" type="text/css" href="css/core/button.css" media="screen" />
<link rel="stylesheet" type="text/css" href="css/mws.theme.css" media="screen" /><!-- JavaScript Plugins -->
<script type="text/javascript" src="js/jquery-1.7.1.min.js"></script><!-- jQuery-UI Dependent Scripts -->
<script type="text/javascript" src="js/jquery-ui-effecs.min.js"></script><!-- Plugin Scripts -->
<script type="text/javascript" src="js/jquery.placeholder.min.js"></script>
<script type="text/javascript" src="js/jquery.validate.min.js"></script><!-- Login Script -->
<script type="text/javascript" src="js/login.js"></script><title><?php echo $title_prefix; ?> - 注册页面</title></head><body>    <div id="mws-login-wrapper">
        <div id="mws-login">
            <h1>会员注册</h1>
            <div class="mws-login-lock"><img src="css/icons/24/locked-2.png" alt="" /></div>
<?php
if ($user -> LoggedIn())
{
echo "<font color='white'>Already logged in, redirecting...</font>";
echo "<meta http-equiv=\"refresh\" content=\"3;url=index.php\">";
die();
}
if (isset($_POST['registerBtn']))
{
if ($securimage->check($_POST['captcha_code']) == false) {
echo '<font color="#e90000">错误码：\ </font>';
}
else {
$username = $_POST['username'];
$password = $_POST['password'];
$rpassword = $_POST['rpassword'];
$email = $_POST['email'];
$errors = array();
$checkUsername = $odb -> prepare("SELECT COUNT(*) FROM `users` WHERE `username` = :username");
$checkUsername -> execute(array(':username' => $username));
$countUsername = $checkUsername -> fetchColumn(0);
if ($checkUsername > 0)
{
$errors['<font color="#e90000">用户名已存在</font>'];
}
if (!ctype_alnum($username) || strlen($username) < 4 || strlen($username) > 15)
{
$errors[] = '<font color="white">Username Must Be  Alphanumberic And 4-15 characters in length</font>';
}
if (!filter_var($email, FILTER_VALIDATE_EMAIL))
{
$errors[] = '<font color="white">邮箱无效</font>';
}
if (empty($username) || empty($password) || empty($rpassword) || empty($email))
{
$errors[] = '<font color="white">请填写有效的邮箱</font>';
}
if ($password != $rpassword)
{
$errors[] = '<font color="white">Passwords do not match</font>';
}
if (empty($errors))
{
$insertUser = $odb -> prepare("INSERT INTO `users` VALUES(NULL, :username, :password, :email, 0, 0, 0, 0)");
$insertUser -> execute(array(':username' => $username, ':password' => SHA1($password), ':email' => $email));
echo '<div class="nNote nSuccess hideit"><p><strong><font color="#00ea2c">成功: </font></strong><font color="#00ea2c">注册用户....</font></p></div><meta http-equiv="refresh" content="3;url=login.php">';
}
else
{
echo '<div class="nNote nFailure hideit"><p><strong>ERROR:</strong><br />';
foreach($errors as $error)
{
echo '-'.$error.'<br />';
}
echo '</div>';
}
}
}
?>
            <div id="mws-login-form">
                <form class="mws-form" method="post">
                    <div class="mws-form-row">
                        <div class="mws-form-item large">
                            <input type="text" name="username" class="mws-login-username mws-textinput required" placeholder="用户名" />
                        </div>
                    </div>
                    <div class="mws-form-row">
                        <div class="mws-form-item large">
                            <input type="password" name="password" class="mws-login-password mws-textinput required" placeholder="密码" />
                        </div>
                    </div>
<div class="mws-form-row">
                        <div class="mws-form-item large">
                            <input type="password" name="rpassword" class="mws-login-password mws-textinput required" placeholder="确认密码" />
                        </div>
                    </div>
<div class="mws-form-row">
                        <div class="mws-form-item large">
                            <input type="text" name="email" class=" mws-textinput required" placeholder="邮箱" />
                        </div>
                    </div>
<div class="mws-form-row">
                        <div class="mws-form-item large">
<img id="captcha" src="captcha/securimage_show.php" alt="CAPTCHA Image" />
<input type="text" class="mws-textinput" style="float:right;margin-top:20px;width:105px;" name="captcha_code" maxlength="6" />
<a style="float:right;margin-top:-52px;width:105px;color:#02a0a2;" href="#" onclick="document.getElementById('captcha').src = 'captcha/securimage_show.php?' + Math.random(); return false">[ 刷新验证码 ]</a>
                        </div>
                    </div>
                    <div class="mws-form-row mws-inset">
                        <ul class="mws-form-list inline">
                            
                        </ul>
                    </div>
                    <div class="mws-form-row">
                        <input type="submit" value="Register" name='registerBtn' class="mws-button blue mws-login-button" /><br />
</a></center>
                    </div>
                </form>
            </div>
        </div>
    </div></body>
</html>