php - 我是 PHP 新手，收到此错误 :

Question

Notice: Use of undefined constant msg - assumed 'msg' in C:\wamp\www\Stanly 1 - Copy\index.php on line 48 Log In

Notice: Use of undefined constant msg - assumed 'msg' in C:\wamp\www\Stanly 1 - Copy\index.php on line 50

Notice: Use of undefined constant msg - assumed 'msg' in C:\wamp\www\Stanly 1 - Copy\index.php on line 50

Notice: Use of undefined constant msg - assumed 'msg' in C:\wamp\www\Stanly 1 - Copy\index.php on line 50

我在网上找不到任何解决方案，所以也许有人可以帮助我。

代码如下:

if($_GET[msg] != ""){ ?>
    <tr>
        <td colspan="2" style="color:#FF0000;" align="center"><?php if ($_GET[msg] == 1) {
    echo "Login failed.";
} elseif ($_GET[msg] == 2) {
    echo "Only Administrator allowed.";
} elseif ($_GET[msg] == 3) {
    echo "You have logged out.";
} ?> </td>
    </tr>
<?php } ?>
    <tr>
        <td width="120" class="style23">Username</td>
        <td width="164" class="style19">&nbsp;<input type="text" name="username" /></td>
    </tr>
    <tr>
        <td class="style23">Password</td>
        <td class="style19">&nbsp;<input type="password" name="password" /></td>
    </tr>
    <tr>
        <td class="style19" colspan="2" align="center">&nbsp;
            <input type="submit" name="login" value="Log in" />
        </td>
    </tr>
</table>

在那之后我所做的(或者至少我正在尝试)是更新 MySQL 中的表。

感谢您花时间回答我。干杯!

Answer 1

您的问题是未加引号的字符串。

$_GET[msg]

应该是

$_GET['msg']

因为您没有使用引号引起来，PHP 会尝试找到常量 msg，但找不到，并将其解释为字符串，这正是您首先想要的。

只要没有定义常量 msg，您的代码就会显示该通知，但会按预期运行。

您的代码变为:

if($_GET['msg'] != ""){ ?>
  <tr>
    <td colspan="2" style="color:#FF0000;" align="center"><?php if($_GET['msg'] == 1){ echo "Login failed."; } elseif($_GET['msg'] == 2){ echo "Only Administrator allowed."; }elseif($_GET['msg'] == 3){ echo "You have logged out."; }?> </td>
  </tr>
<?php } ?>
<tr>
  <td width="120" class="style23">Username</td>
  <td width="164" class="style19">&nbsp;<input type="text" name="username" /></td>
</tr>
<tr>
  <td class="style23">Password</td>
  <td class="style19">&nbsp;<input type="password" name="password" /></td>
</tr>
<tr>
 <td class="style19" colspan="2" align="center">&nbsp;
   <input type="submit" name="login" value="Log in" />
 </td>
</tr>