这个问题在这里已有答案：
我在这里有一点错误。我正在尝试发送标题时收到此消息：
警告：无法修改标头信息 - 已在第34行的E：\ xampp \ htdocs \ kursova1 \ pages \ login.php中发送的标头（输出从E：\ xampp \ htdocs \ kursova1 \ layout \ content.php：1开始）
在content.php中，这是我的代码如下：
<div id="body-wrap" data-background="url(css/images/rubber_grip.png) repeat" class="cactus-box video-v2-setbackground ">
<div id="wrap">
<header id="header-navigation">
<!--Navigation style-->
<div class="cactus-nav-control cactus-nav-style-3">
<div class="cactus-nav-main dark-div dark-bg-color-1" >
<div class="cactus-container padding-30px ">
<div class="cactus-row magin-30px">
<!--nav left-->
<div class="cactus-nav-left">
<!--logo-->
<div class="cactus-logo navigation-font">
<a href="http://videopro.cactusthemes.com/v2/">
<img src="http://cdn.cactusthemes.com/wp-content/uploads/2016/05/01-videopro-logo-dark-1x.png" alt="VideoPro" title="VideoPro" class="cactus-img-logo">
<img src="http://cdn.cactusthemes.com/wp-content/uploads/2016/05/01-videopro-logo-dark-1x.png" alt="VideoPro" title="VideoPro" class="cactus-img-logo cactus-img-sticky">
</a>
</div><!--logo-->
<!--header search-->
<div class="cactus-header-search-form search-box-expandable">
<aside id="advance_search_form-2" class="widget widget-asf">
<style>#advance_search_form-2 .ct-sub-w-title{color:FFFFFF !important; background:FF0000 !important}</style>
<div class="widget-inner">
<div class="cactus-main-menu cactus-open-search-mobile navigation-font">
<ul>
<li><a href=""><i class="fa fa-search"></i></a></li>
</ul>
</div>
<form role="search" method="get" id="searchform">
<div>
<span class="searchtext">
<input id="search1" onchange="" type="text" value="" name="s" class="ss" autocomplete="off" placeholder="Enter Keyword"/>
<span class="suggestion"><!-- --></span>
<i class="fa fa-search" aria-hidden="true"></i>
</span>
<input type="hidden" name="video_only" value="1" />
</div>
</form>
</div>
</aside>
</div><!--header search-->
</div> <!--nav left-->
<!--nav right-->
<div class="cactus-nav-right">
<div class="cactus-main-menu cactus-open-menu-mobile navigation-font">
<ul>
<li><a href="javascript:;"><i class="fa fa-bars"></i>MENU</a></li>
</ul>
</div>
<div class="cactus-main-menu cactus-user-login navigation-font">
<ul>
<li>
<a href="index.php?p=login"><i class="fa fa-user"></i> Вход</a>
</li>
</ul>
</div>
</div><!--nav right-->
</div>
</div>
</div>
</div>
<div class="cactus-nav-control cactus-nav-style-3"> <!--add Class: cactus-nav-style-3-->
<div class="cactus-nav-main dark-div dark-bg-color-1">
<div class="cactus-container padding-30px ">
<!--Menu Down-->
<div class="cactus-row magin-30px">
<!--nav left-->
<div class="cactus-nav-left cactus-only-main-menu">
<!--main menu / megamenu / Basic dropdown-->
<div class="cactus-main-menu navigation-font">
<ul class="nav navbar-nav">
<!-- <li id="mega-menu-item-1589" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="index.php?p=home"><i class="fa fa-home"></i> Начало</a>
</li>
<li id="mega-menu-item-773" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="#"><i class="fa fa-bars"></i> Features</a>
<li id="mega-menu-item-3236" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="index.php?p=forum"><i class="fa fa-users"></i> Форум</a>
</li>
<li id="mega-menu-item-3279" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="#"><i class="fa fa-search-plus"></i> Browse</a>
</li>
<li id="mega-menu-item-3456" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="#"><i class="fa fa-dollar"></i> Premium Content</a>
</li> -->
<?php
$pages = array(
"home" => "Начало",
"categories" => "Категории",
"forum" => "Форум",
"upload" => "Качи видео",
"contact" => "Контакти",
);
$p = (isset($_GET['p'])) ? $_GET['p'] : "";
foreach ($pages as $url => $label) {
echo '<li id="mega-menu-item-1589" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style ';
if ($p == $url) {
echo 'current-menu-ancestor current-menu-parent';
}
echo '"><a href="index.php?p=' . $url . '"><i class="fa fa-home"></i>' . $label . '</a></li>';
}
?>
<li id="mega-menu-item-3189" class=" menu-item menu-item-type-post_type menu-item-object-page level0 main-menu-item list-style"><a href="index.php?p=register"><i class="fa fa-arrow-circle-o-right"></i> Регистрация</a>
</li>
</ul>
</div><!--main menu-->
</div><!--nav left-->
</div>
<!--Menu Down-->
</div>
</div>
</div>
<!--Navigation style-->
</header>
<header id="header-navigation" class="sticky-menu">
<!--Navigation style-->
<div class="cactus-nav-control">
<div class="cactus-nav-main dark-div dark-bg-color-1">
<div class="cactus-container padding-30px">
<div class="cactus-row magin-30px reset-default-nav">
<!--nav left-->
<div class="cactus-nav-left">
<!--logo-->
<div class="cactus-logo navigation-font">
<a href="http://videopro.cactusthemes.com/v2/">
<img src="http://cdn.cactusthemes.com/wp-content/uploads/2016/05/01-videopro-logo-dark-1x.png" alt="VideoPro" title="VideoPro" class="cactus-img-logo">
<img src="http://cdn.cactusthemes.com/wp-content/uploads/2016/05/01-videopro-logo-dark-1x.png" alt="VideoPro" title="VideoPro" class="cactus-img-logo cactus-img-sticky">
</a>
</div><!--logo-->
<!--header search-->
<div class="cactus-header-search-form search-box-expandable">
<aside id="advance_search_form-2" class=" widget widget-asf">
<style>#advance_search_form-2 .ct-sub-w-title{color:FFFFFF !important; background:FF0000 !important}</style>
<div class="widget-inner">
<div class="cactus-main-menu cactus-open-search-mobile navigation-font">
<ul>
<li><a href="javascript:;"><i class="fa fa-search"></i></a></li>
</ul>
</div>
<form role="search" onsubmit="if(jQuery('.ss',jQuery(this)).val() == '' || jQuery('.ss',jQuery(this)).val() == '') return false;" method="get" id="searchform" action="http://videopro.cactusthemes.com/v2/">
<div>
<span class="searchtext">
<input id="search2" onchange="updateSearch(this, jQuery('#search1'))" type="text" value="" onfocus="if(this.value == '') this.value = '';" onblur="if(this.value == '') this.value=''" name="s" class="ss" autocomplete="off" placeholder="Enter Keyword">
<span class="suggestion" style="max-height: 145px;"><!-- --></span>
<i class="fa fa-search" aria-hidden="true"></i>
</span>
<input type="hidden" name="video_only" value="1">
</div>
</form>
</div>
</aside>
</div>
<div class="cactus-main-menu cactus-open-menu-mobile navigation-font">
<ul>
<li>
<a href="javascript:;"><i class="fa fa-bars"></i></a>
</li>
</ul>
</div><!--header search-->
</div> <!--nav left-->
<!--nav right-->
<div class="cactus-nav-right">
<div class="cactus-main-menu cactus-open-menu-mobile navigation-font">
<ul>
<li><a href="javascript:;"><i class="fa fa-bars"></i>MENU</a></li>
</ul>
</div>
<div class="cactus-main-menu cactus-user-login navigation-font">
<ul>
<li>
<a href="http://videopro.cactusthemes.com/v2/account-2/?redirect_to=http%3A%2F%2Fvideopro.cactusthemes.com%2Fv2"><i class="fa fa-user"></i> Вход</a>
</li>
</ul>
</div>
</div><!--nav right-->
<div class="cactus-nav-left cactus-only-main-menu">
<!--main menu / megamenu / Basic dropdown-->
<div class="cactus-main-menu navigation-font">
<ul class="nav navbar-nav">
<!-- <li id="mega-menu-item-1589" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="index.php?p=home" class="cactus-hasIcon"><i class="fa fa-home"></i> Начало</a>
</li>
<li id="mega-menu-item-773" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="#" class="cactus-hasIcon"><i class="fa fa-bars"></i> Features</a>
</li>
<li id="mega-menu-item-3236" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="index.php?p=forum" class="cactus-hasIcon"><i class="fa fa-users"></i> Форум</a>
</li>
<li id="mega-menu-item-3279" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="#" class="cactus-hasIcon"><i class="fa fa-search-plus"></i> Browse</a>
</li>
<li id="mega-menu-item-3456" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style"><a href="#" class="cactus-hasIcon"><i class="fa fa-dollar"></i> Premium Content</a>
</li> -->
<?php
$pages = array(
"home" => "Начало",
"categories" => "Категории",
"forum" => "Форум",
"upload" => "Качи видео",
"contact" => "Контакти",
);
$p = (isset($_GET['p'])) ? $_GET['p'] : "";
foreach ($pages as $url => $label) {
echo '<li id="mega-menu-item-1589" class=" menu-item menu-item-type-custom menu-item-object-custom menu-item-has-children parent level0 has-sub dropdown main-menu-item list-style ';
if ($p == $url) {
echo 'current-menu-ancestor current-menu-parent';
}
echo '"><a class="cactus-hasIcon" href="index.php?p=' . $url . '"><i class="fa fa-home"></i>' . $label . '</a></li>';
}
?>
<li id="mega-menu-item-3189" class=" menu-item menu-item-type-post_type menu-item-object-page level0 main-menu-item list-style"><a href="index.php?p=register"><i class="fa fa-arrow-circle-o-right"></i> Регистрация</a>
</li>
</ul>
</div><!--main menu-->
</div>
</div>
</div>
</div>
</div>
</header>
<div id="cactus-body-container">
<?php
$pages_dir = 'pages';
if (!empty($_GET['p'])) {
$pages = scandir($pages_dir, 0);
unset($pages[0], $pages[1]);
$p = $_GET['p'];
if (in_array($p.'.php', $pages)) {
include($pages_dir.'/'.$p.'.php');
} else {
echo 'Sorry, page not found.';
}
} else {
include($pages_dir.'/home.php');
}
?>
</div><!-- container -->
如你所见，我没有任何空格，我也没有看到任何错误。
这是我的登录页面，我正在尝试发送它：
<div class="cactus-sidebar-control ">
<div class="cactus-container ">
<div class="cactus-row">
<div class="main-content-col">
<div class="main-content-col-body">
<div class="single-page-content">
<article class="cactus-single-content">
<div class="single-page-content single-content">
<article id="post-3165" class="post-3165 page type-page status-publish hentry cactus-single-content">
<div class="body-content">
<!--Content-->
<div class="ms-account-wrapper videopro-cusom-cl">
<div class="ms-membership-form-wrapper">
<div class="ms-login-form">
<?php
$error = '';
if (isset($_POST['login'])) {
$username = $_POST['username'];
$password = $_POST['password'];
if (empty($username) or empty($password))
{
$error = 'Въведените данни не отговарят на изискванията!';
} else {
$query = mysqli_query($conn, "SELECT * FROM users WHERE username='$username' AND password='".md5($password)."'");
$rows = mysqli_num_rows($query);
if ($rows == 1) {
$row = mysqli_fetch_assoc($query);
$_SESSION['uid'] = $row['UserID'];
$_SESSION['username'] = $row['username'];
header('Location: ./index.php?p=welcome');
} else {
$error = "Въведените данни са грешни!";
}
}
}
?>
<p><?php echo $error; ?></p>
<form name="loginform" id="loginform" action="" method="post" class="ms-form ms-form-login ms-no-labels autofocus" style="">
<div class="form">
<label>Потребителско име:</label>
<input type="text" name="username" id="user_login" class="input focus" value="" size="10" placeholder="Username">
<label>Парола:</label>
<input type="password" name="password" id="user_pass" class="input" value="" size="20" placeholder="Password">
<div class="nav">
<p>
<a class="lost" href="#lostpassword">
Забравена парола?
</a>
</p>
</div>
<p class="login-remember ms-field">
<input name="rememberme" type="checkbox" id="rememberme" value="forever" />
<label for="rememberme"> Запомни ме</label>
</p>
<p class="login-submit">
<input type="submit" name="login" id="wp-submit" class="button-primary" value="Вход" />
</p>
</div>
</form>
</div>
<a href="index.php?p=register">Регистрация?</a>
</div>
</div>
<div class="ms-account-wrapper"> </div>
<hr />
</div>
</article>
</div>
</article>
</div>
</div>
</div>
</div>
</div>
如果有人知道我的错误在哪里，我会很高兴听到它。顺便说一句，如果我把我的登录页面中的PHP代码放在content.php的开头，它工作正常，但我不认为它必须在那里。谢谢！
将此代码放在具有标题的非常php页面的顶部
<?php
ob_start();
// your code here
?>