headers_sent

(PHP 4, PHP 5, PHP 7, PHP 8)

headers_sent — 检测消息头是否已经发送

说明

headers_sent(string &$filename = null, int &$line = null): bool

检测消息头是否已经发送。

消息头已经发送时，就无法通过 header() 添加更多头字段。使用此函数起码可以防止收到跟消息头相关的错误。另一个解决方案是用输出缓冲。

参数

filename: 若设置了可选参数 filename 和 line，headers_sent() 会把 PHP 文件名放在 filename 变量里，输出开始的行号放在 line 变量里。

注意:
如果在执行 PHP 源文件之前已经开始输出（例如由于启动错误），则 filename 参数将被设置为空字符串。
line: 输出开始的行号。

返回值

消息头未发送时，headers_sent() 返回 false，否则返回 true。

示例

示例 #1 使用 headers_sent() 的例子

<?php

// 没有消息头就发送一个
if (!headers_sent()) {
    header('Location: http://www.example.com/');
    exit;
}

// 使用 file 和 line 参数选项的例子
// 注意 $filename 和 $linenum 用于下文中使用
// 所以不要提前为它们赋值
if (!headers_sent($filename, $linenum)) {
    header('Location: http://www.example.com/');
    exit;

// 很有可能在这里触发错误
} else {

    echo "Headers already sent in $filename on line $linenum\n" .
          "Cannot redirect, for now please click this <a " .
          "href=\"http://www.example.com\">link</a> instead\n";
    exit;
}

?>

注释

注意:
数据头只会在SAPI支持时得到处理和输出。

参见

ob_start() - 打开输出控制缓冲
trigger_error() - 产生一个用户级别的 error/warning/notice 信息
headers_list() - 返回已发送的 HTTP 响应头（或准备发送的）
header() - 发送原生 HTTP 头中有更多相关细节的讨论。

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用户贡献的备注 7 notes

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yesmarklapointe at hotmail dot com ¶

16 years ago

I was having trouble getting my mind around the concepts involved. Here is my dilemma and the conclusion I reached in case recounting them can help others:

I am using WAMPserver: PHP 5.2.6, and Apache 2.2.8 on Windows XP SP3. If it matters to your duplication, 
I found two php.ini files in WAMPserver where output_buffering had been set to 4096. I changed them to OFF for this testing. 

Here is how you can replicate what I am experiencing: With IE 7.0 go to Tools ... Display ieHTTPheaders ... and run the following script repeatedly and watch what happens:

<?php
header( 'Expires: Mon, 26 Jul 1998 05:00:00 GMT' );
//var_dump(headers_sent());
//print("whatever");
//flush();
//echo "whatever";
var_dump(headers_sent());
?>

Result: the final var_dump of the headers_sent() function will 
always return FALSE unless any one or  more of the commented lines above it are uncommented. Uncommenting the statements allows an output to be sent to the user not just to their browser, after which the final var_dump will return TRUE. What I found confusing was that the ieHTTPheaders tool shows that the header is being sent to the user's browser even when all the output lines are commented out. So why does headers_sent() return FALSE in this case? Because you can keep sending other headers. The headers_sent function is meant to alert one to when no further headers can be sent. My testing shows it does not return true unless some other output is also sent after the header, thereby signaling that  "Headers have been sent and concluded with user output. NOW you can't send any more headers."

Someone else worked his way through this problem in a (false) bug report: http://bugs.php.net/bug.php?id=30264
Here is the relevant part of the reply from the pro:
"When you use compression the entire page is buffered in memory, until end of the request. Consequently you can send headers at any time because no data is being actually sent to user when you print it. Until PHP actually decides to send any page output to the user you can still send additional headers which is why the headers_sent() function is returning false. It will return true, indicating that headers have been sent  only at a time when output began going to the user and you no longer can send any additional headers."

So in summary, my point is that there is a difference between headers being sent only to the browser (which can be followed by other headers) vs. headers being sent and concluded by output for the user. The function should have been given a more clear name like headers_concluded().

down

Jakob B. ¶

18 years ago

<?php
function redirect($filename) {
    if (!headers_sent())
        header('Location: '.$filename);
    else {
        echo '<script type="text/javascript">';
        echo 'window.location.href="'.$filename.'";';
        echo '</script>';
        echo '<noscript>';
        echo '<meta http-equiv="refresh" content="0;url='.$filename.'" />';
        echo '</noscript>';
    }
}
redirect('http://www.google.com');
?>

down

php at fufachew dot REMOVEME dot com ¶

20 years ago

RE: antti at haapakangas dot net's post

I've changed the code so $_SERVER['SERVER_NAME'] is used if $_SERVER['HTTP_HOST'] is not set.  $_SERVER['SERVER_NAME'] doesn't meet my needs, but I suppose it's good to fall back on it.  I've also fixed a problem in the meta refresh line - it was missing the "url=" part of the content attribute.

<?php
function server_url()
{   
    $proto = "http" .
        ((isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == "on") ? "s" : "") . "://";
    $server = isset($_SERVER['HTTP_HOST']) ?
        $_SERVER['HTTP_HOST'] : $_SERVER['SERVER_NAME'];
    return $proto . $server;
}
    
function redirect_rel($relative_url)
{
    $url = server_url() . dirname($_SERVER['PHP_SELF']) . "/" . $relative_url;
    if (!headers_sent())
    {
        header("Location: $url");
    }
    else
    {
        echo "<meta http-equiv=\"refresh\" content=\"0;url=$url\">\r\n";
    }
}
?>

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trevize (shtrudel) gmail.com ¶

19 years ago

Note that in IIS (or at least the version that comes with W2K server), the server seems to do some buffering, so even if you output someting or cause a warning, the value of headers_sent() may be false because the headers haven't been sent yet.

So it's not a safe way to know if warnings have been encountered in your script.

down

collectours at free dot fr ¶

17 years ago

In response to K.Tomono and alexrussell101 at gmail dot com :

Yes,
headers_sent() will return false, even if you sent something to the ouptut using print() or header() , if output_buffering is different from Off in you php.ini, and the length of what you sent does not exceed the size of output_buffering.

To test it, try this code with  these values in php.ini
1) output_buffering=32
2) output buffering = 4096

[code]
<?php
    echo "Yo<br />";
    echo "Sent:",headers_sent(),"<br />";
    echo "enough text to feed the buffer until it overflows ;-)<br />";
    echo "Sent:",headers_sent(),"<br />";
?>
[/code]

then put 
3) output buffering = Off
and try this code
[code]
<?php
    echo "Yo<br />";
    echo "Sent:",headers_sent(),"<br />";
?>
[/code]
which will this time unconditionnally say that headers were sent.

This is noticed in php.ini comment :
"Output buffering allows you to send header lines (including cookies) even after you send body content, in the price of slowing PHP's output layer a bit."

Note : This is completly independant of implicit_flush tuning.

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rojasredes at gmail dot com ¶

6 years ago

Remember to save source code file *.php with UTF-8 without BOM.
If saved with UTF-8 BOM the next code allways return "true":

<?php
if headers_sent($source,$numline)
{   
    die("true");
}
else
{
    die("false");
}
?>

Even when $numline is 1, and there is any character before <?php
and after ?>

Save file without BOM and everything will be ok, then.

down

Jase ¶

15 years ago

This is becoming annoying the amount of posts to try and describe the behaviour of headers

Headers appear first in the data sent to the user's browser

if headers have been called using the header() function but no data has been sent to the output buffer (using echo, readfile etc), then the headers are sent at the end of script execution otherwise they are sent when the buffer reaches it's limit and emptied

simple

this means that headers_sent() will return false if nothing is sent to the output buffer because the headers are being sent at the end of the script

This is not a bug either, this is expected behaviour. Keeping headers until forced to send them out is a good idea because certain measures can be taken like prevention of meta injection etc (option in header() to replace headers that have not yet been sent)

besides, headers_sent() is good for when you try and send headers but the output buffer has already been emptied (in cases of php error handling for example). Obviously if the buffer has emtpied, sending headers won't work.