PHP - 未定义的变量

我正在做一些练习PHP 和 MySQL 初学者塔克先生。

在他的示例中一切正常，但在我的电脑上出现错误：

注意：未定义变量：passwordRetrieved 在 C:\wamp\www\loginForm.php 第 39 行

此示例的完整 PHP 代码：

• 请注意该表确实存在，并且密码、数据库连接等都是正确的

  <?php
      {
          // Secure Connection Script
          include('htconfig/dbConfig.php');
  
          $dbSuccess = false;
          $dbConnected = mysql_connect($db['hostname'], $db['username'], $db['password']);
  
          if ($dbConnected) {
              $dbSelected = mysql_select_db($db['database'], $dbConnected);
              if ($dbSelected) {
                  $dbSuccess = true;
              }
          }
          // END Secure Connection Script
      }
  
      $thisScriptName = "loginForm.php";
  
      echo '<h2>Login Form </h2>';
  
      $username = $_POST['username'];
  
      if(isset($username)) {
          $password = $_POST['password'];
          echo "username = " . $username . "<br />";
          echo "password = " . $password . "<br />";
  
          {
              // SELECT password for this user from the DB and see it it matches
              $tUser_SQLselect = "SELECT password FROM tUser ";
              $tUser_SQLselect .= "WHERE username = '" . $username . "' ";
  
              $tUser_SQLselect_Query = mysql_query($tUser_SQLselect);
              while ($row = mysql_fetch_array($tUser_SQLselect_Query, MYSQL_ASSOC)) {
                  $passwordRetrieved = $row['password'];
              }
              mysql_free_result($tUser_SQLselect_Query);
  
              echo "passwordRetrieved = ".$passwordRetrieved."<br />";
  
              if (!empty($passwordRetrieved) AND ($password == $passwordRetrieved)) {
  
                  echo "YES. Password matches.<br /><br />";
                  echo '<a href="' . $thisScriptName . '">Logout</a>';
              }
              else {
                  echo "Access denied.<br /><br />";
                  echo '<a href="' . $thisScriptName . '">Try again</a>';
              }
          }
      }
      else {
  
          echo '<form name="postLoginHid" action="' . $thisScriptName . '" method="post">';
                  echo '
                      <P>User name:
                      <INPUT TYPE=text NAME=username value=""></P>
                      <P>Password:
                      <INPUT TYPE=password NAME=password value=""></P>
                      <input type="submit"  value="Login" />
                  ';
          echo '</form>';
      }
  
      echo '<h2>--------- END Login Form --------</h2>';
  ?>
  

就在之前while在设置变量 $password Retrieved 的地方声明它，因此它应该如下所示：

$tUser_SQLselect_Query = mysql_query($tUser_SQLselect);
$passwordRetrieved = "";


while ($row = mysql_fetch_array($tUser_SQLselect_Query, MYSQL_ASSOC)) {
    $passwordRetrieved = $row['password'];
}