检查用户是否是管理员PHP

我有一个具有以下结构的用户表：

id
用户名
密码
经销商（管理员）

现在我想在登录时检查用户是否是经销商，经销商可以持有0（普通用户）或1（管理员）的值，但我不知道如何做到这一点（我是PHP新手）。

这是登录表单：

    <form action="index.php?action=login" method="post" style="width: 50%;">
        <input type="hidden" name="login" value="true" />
<?php if ( isset( $results['errorMessage'] ) ) { ?>
        <div class="errorMessage"><?php echo $results['errorMessage'] ?></div>
<?php } ?>
        <ul>
          <li>
            <label for="username">Username</label>
            <input type="text" name="username" id="username" placeholder="Uw gebruikersnaam" required autofocus maxlength="20" />
          </li>
          <li>
            <label for="password">Password</label>
            <input type="password" name="password" id="password" placeholder="Uw wachtwoord" required maxlength="20" />
          </li>
        </ul>
        <div class="buttons">
          <input type="submit" name="login" value="Login" />
        </div>
      </form>

这是登录功能：

    function login() {
  $results = array();
  $results['pageTitle'] = "Admin Login | Gemeente Urk";
  $host = "localhost";
  $mysqluser = "root";
  $mysqlpass = "usbw";
  $db = "wagenpark";
  mysql_connect($host, $mysqluser, $mysqlpass);
  mysql_select_db($db);
  if ( isset( $_POST['login'] ) ) {
      $gebruiker = $_POST['username'];
      $wachtwoord = $_POST['password'];
      $sql = "SELECT * FROM users WHERE username='".$gebruiker."' AND password='".$wachtwoord."' LIMIT 1";
      $res = mysql_query($sql) or die (mysql_error());
      if (mysql_num_rows($res) == 1) {
          $_SESSION['username'] = $gebruiker;
          header( "Location: index.php" );
    } else {
      // Login failed: display an error message to the user
      $results['errorMessage'] = "Incorrect username or password. Please try again.";
      require( TEMPLATE_PATH . "/admin/loginForm.php" );
    }
  } else {
    // User has not posted the login form yet: display the form
    require( TEMPLATE_PATH . "/admin/loginForm.php" );
  }
}

已经谢谢了。