警告:无法修改标头信息 - 标头已发送 PHP 错误程序 - Warning: Cannot modify header information - headers already sent PHP ERRORR

Warning: Cannot modify header information - headers already sent PHP ERRORR

我有这个错误：

警告：无法修改标头信息 - 标头已由发送（输出开始于 D：''xamp''htdocs''talcioc''login.php：46） in D：''xamp''htdocs''talcioc''login.php 在第 77 行

这就是代码（索引.php）：

    <?php
if(isset($_SESSION['IS_LOGEDIN']) == 'Y')
{
header("location:index.php");
exit();
}
?>
<form action="" method="post">
  <table width="524" border="0" style="-webkit-border-radius:9px;
    -moz-border-radius:9px;
    -ms-border-radius:9px;
    border-radius:9px; background-image:url(images/bgtable.png);" align="center">
    <tr>
      <td width="514"><center><img src="pics/admin2.png" width="128" height="128" /></center></td>
    </tr>
    <tr>
      <td><p>&nbsp;</p></td>
    </tr>
  </table>
  <p>&nbsp;</p>
  <table width="304" border="0" style="background-color:#000; -webkit-border-radius:9px;
    -moz-border-radius:9px;
    -ms-border-radius:9px;
    border-radius:9px;" align="center">
    <tr>
      <td width="78">Username:</td>
      <td width="210"><input type="text" name="myusername" /></td>
    </tr>
    <tr>
      <td>Password:</td>
      <td><input type="password" name="mypassword" /></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td><input type="submit" value="Login!" name="submit" /></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td><img src="images/key.png" alt="" width="16" height="16" border="0" align="bottom" />&nbsp;<a href="index.php?pagina=forgotpass">Ti-ai uitat parola?</a></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td>
      <?php 
if(isset($_POST['submit']))
{
if(!isset($_SESSION['IS_LOGEDIN']))
{
    $_SESSION['IS_LOGEDIN'] = 'N';
}
// username and password sent from form
$myusername=$_POST['myusername'];
$mypassword=$_POST['mypassword'];
// To protect MySQL injection (more detail about MySQL injection)
$myusername = stripslashes(mysql_real_escape_string($myusername));
$mypassword = stripslashes(mysql_real_escape_string($mypassword));
$encrypted_mypassword=md5($mypassword);
$sql="SELECT * FROM administratori WHERE username='$myusername' and password='$encrypted_mypassword'";
$result=mysql_query($sql);

// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row
if($count==1){
// Register $myusername, $mypassword and redirect to file "login_success.php"
$_SESSION['myusername'] = $myusername;
$_SESSION['IS_LOGEDIN'] = 'Y';
//$lastlogin = $_POST['date("d/m/y  H:i:s", time()+25200)'];
header("location:administrare.php");
}
else {
echo '<img src="images/delete.png" width="16" height="16" />' . '&nbsp;' .'User sau Password gresit!';
}
}
?>
</td>
    </tr>
  </table>
</form>

当你说： header("location:administrare.php");这很糟糕，因为在header之前，缓冲区中不应该有任何输出。

检查"已登录"条件后，将整个if(isset($_POST['submit']))条件移动到文件顶部。

header函数必须在

任何输出之前调用。

文件的结构必须是：

<?php
if(isset($_SESSION['IS_LOGEDIN']) == 'Y') {
    header("location:index.php");
    exit();
}
if (isset($_POST['submit'])) {
    // process form
}
?>
<form>
    ...
</form>