php中会话变量存在未定义索引错误

我得到的错误如下 注意：未定义索引：登录第19行的C:\xampp\htdocs\industane\includes\top-header.php 下面提到了我的top-header.php文件：

<div class="top-banner-container">
      <a class="routine-banner-link" href="#"> </a>
    </div>
    <div class="container-fluid top-lighthouse">
      <nav class="navbar navbar-default">
        <div class="container top-lighthouse-container">
          <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
              <span class="sr-only">Toggle navigation</span>
              <span class="icon-bar"></span>
              <span class="icon-bar"></span>
              <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand visible-xs" href="#">Menu</a>
          </div>
          <div id="navbar" class="navbar-collapse collapse">
            <ul  class="nav navbar-nav navbar-right">

                        <?php if(strlen($_SESSION['login']))
            {   ?>
              <li><a href="#"><i class="icon fa fa-user"></i>Welcome -<?php echo htmlentities($_SESSION['username']);?></a></li>
                <?php } ?>
              <li class="navbar-border"><a class="text-muted navbar-position" href="#">Buyer Protection</a></li>
              <li class="dropdown navbar-border">
                <a href="#" class="navbar-position text-muted dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Help <span class="glyphicon glyphicon-menu-down"></span></a>
                <ul class="dropdown-menu">
                  <li><a class="text-muted navbar-position" href="#">Customer Service</a></li>
                  <li><a class="text-muted navbar-position" href="#">Disputes & Reports</a></li>
                </ul>
              </li>
              <li class="navbar-border"><a class="text-muted navbar-position" href="#">Mobile</a></li>
              <li class="dropdown navbar-border">
                <a href="#" class="text-muted dropdown-toggle navbar-position" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Ship to <span class="glyphicon glyphicon-globe"></span> /  USD<span class="glyphicon glyphicon-menu-down"></span></a>
                <ul class="dropdown-menu">
                  <li><a class="text-muted navbar-position" href="#">Customer Service</a></li>
                  <li><a class="text-muted navbar-position" href="#">Disputes & Reports</a></li>
                </ul>
              </li>
              <li class="dropdown">
                <a href="#" class="text-muted dropdown-toggle navbar-position" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Language<span class="glyphicon glyphicon-menu-down"></span></a>
                <ul class="dropdown-menu">
                  <li><a class="text-muted navbar-position" href="#">Customer Service</a></li>
                  <li><a class="text-muted navbar-position" href="#">Disputes & Reports</a></li>
                </ul>
              </li>
            </ul>
          </div><!--/.nav-collapse -->
        </div><!--/.container-fluid -->
      </nav>
    </div>

切换导航

我想不出怎么解决它，需要帮助

但在您的情况下，您可以解决您的错误，如下面所述，或只是关闭通知，如错误报告（error_reporting（）&~E_notice）

需要替换为

if(isset($_SESSION['login']))

---

我认为它在SO中的最大重复问题数是“未定义索引”。已经这样做了，现在正在工作

if(strlen($_SESSION['login']))

if(isset($_SESSION['login']))