$errors[] not returning

我正在为我的网站构建一个PHP登录和注册系统。如果表单字段为空，则编码调用使用$errors[]，并使用函数检查数据库中是否存在输入用户名。我没有收到任何错误，即使我没有输入任何信息。

login.php

<?php
include 'cic/initalize.php';
if (user_exists('cassey') === true) {
    echo 'exists';
}
die();
if (empty($_POST) === false) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    if (empty($username) === true || empty($password) === true) {
        $errors[] = 'Please provide your username and password.';
    } else if (user_exists($username) === false) {
        $errors[] = 'We can''t find the username entered, please enter a valid 
        username or register to continue';
        }
}
?>

clients.php（用户）

<?php
function user_exists($username) {
    $username = sanitize($username);
    return (mysql_result(mysql_query("SELECT userId FROM clients WHERE 
    username = '$username'"), 0) === 1) ? true : false; 
}
?>

调用login.php的代码

 <div class="widget">
      <h2>Login | Register</h2>
 </div><!--End widget class tag-->
 <div class="inside">
    <form action="login.php" method="post">
       <ul id="logIn">
          <li>
            Username:<br/>
            <input type="text" name="username"/>
          </li>
          <li>
            Password:<br/>
            <input type="password" name="password"/>
          </li>
          <li>
            <input type="submit" value="Login"/>
          </li>
          <li>
            <a href="register.php">Register</a>
          </li>
       </ul>
    </form>
 </div><!--End inside class tag-->