If you got this message: "Warning: Cannot modify header information - headers already sent by ...."
如果在执行php程序时看到这条警告:"Warning: Cannot modify header information - headers already sent by ...."
Few notes based on the following user posts:
有以下几种解决方法:
1. Blank lines (空白行):
Make sure no blank line after <?php ... ?> of the calling php script.
检查有<?php ... ?> 后面没有空白行，特别是include或者require的文件。不少问题是这些空白行导致的。
2. Use exit statement (用exit来解决):
Use exit after header statement seems to help some people
在header后加上exit();
header ("Location: xxx");
exit();
3. PHP has this annoying problem, if your HTML goes before any PHP code or any header modification before redirecting to certain page, it'll said "Warning: Cannot modify header information - headers already sent by ...." Basically anytime you output to browser, the header is set and cannot be modified. So two ways to get around the problem:
3a. Use Javascript (用Javascript来解决):
<? echo "<script> self.location(\"file.php\");</script>"; ?>
Since it's a script, it won't modify the header until execution of Javascript.
可以用Javascript来代替header。但是上面的这段代码我没有执行成功... 另外需要注意，采用这种方法需要浏览器支持Javascript.
3b. Use output buffering (用输出缓存来解决):
<?php ob_start(); ?>
... HTML codes ...
<?php
... PHP codes ...
header ("Location: ....");
ob_end_flush();
?>
This will save the output buffer on server and not output to browser yet, which means you can modify the header all you want until the ob_end_flush() statement. This method is cleaner than the Javascript since Javascript method assumes the browser has Javascript turn on. However, there are overhead to store output buffer on server before output, but with modern hardware I would imagine it won't be that big of deal. Javascript solution would be better if you know for sure your user has Javascript turn on on their browser.
在后台管理或者有时候在论坛，点击一个页面，页顶会出现
Warning: Cannot modify header information - headers already sent by....
这类语句，造成这个原因是因为setcookie语句的问题。
cookie本身在使用上有一些限制，例如：
1.呼叫setcookie的叙述必须放在<html>标签之前
2.呼叫setcookie之前，不可使用echo
3.直到网页被重新载入后，cookie才会在程式中出现
4.setcookie函数必须在任何资料输出至浏览器前，就先送出
5.……
基於上面这些限制，所以执行setcookie()函数时，常会碰到"Undefined index"、"Cannot modify header information - headers already sent by"…等问题，解决"Cannot modify header information - headers already sent by"这个错误的方法是在产生cookie前，先延缓资料输出至浏览器，因此，您可以在程式的最前方加上ob_start();这个函数。这样就可以解决了。
4.set output_buffering = On in php.ini (开启php.ini中的output_buffering )
set output_buffering = On will enable output buffering for all files. But this method may slow down your php output. The performance of this method depends on which Web server you're working with, and what kind of scripts you're using.
这种方法和3b的方法理论上是一样的。但是这种方法开启了所有php程序的输出缓存，这样做可能影响php执行效率，这取决于服务器的性能和代码的复杂度。
$fp = fopen($file, "r");
while (!feof($fp))
{
echo fread($fp, 65536);
flush(); // this is essential for large downloads
}
fclose($fp);
?>
运行了一下发现不行，一直报错：Warning: Cannot modify header information - headers already sent by (output started at E:\xampp\htdocs\test\downloadfile\file_download.php:1) in E:\xampp\htdocs\test\downloadfile\file_download.php on line 3
方法二：
解 决Warning: Cannot modify header information - headers already sent by ...... 前几天装了个php的大头贴系统测试，发现报错Warning: Cannot modify header information - headers already sent by ......
今天又装openads，还是出现这个问题。怒了。上网找了半天，有人说要在文件开头写上
ob_start();
失败。
后来打开 php.ini 然后把 output_buffering 设为 on 。重起appache，OK。看来这才是解决办法。