转：

从一台机子上移植完全相同的代码到另外一台机器上，用的都是lamp配置。但是在运行新代码的时候waring如下：
PHP Warning:  Cannot modify header information - headers already sent by (output started at /ms/wwwroot/huodongv2/layout/layout_event_detail.php:301) in /ms/wwwroot/common/header.php on line 58, referer: http://huodong.myspace.cn/

google找到了以下这篇文章，总结起来，就是因为在header前多了一些空白输出。可以通过删去代码中的空白输出、ob_clean()、修改php.ini中的output_buffering等方法以解决。

ps：这个问题的一个表象就是在页面源代码的开头有空白。

以下为转载。

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转自：http://www.phpchina.com/26247/viewspace_8026.html

昨晚在转换编码的时候，有一个页面需要在head中申明utf-8的编码，但是这与程序中的一处header产生了冲突。google了一下，找到几种解决方法，翻译整理一下：

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 of the calling php scrīpt.
检查有 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 Javascrīpt (用Javascrīpt来解决):
self.location( file.php );"; ?>
Since it s a scrīpt, it won t modify the header until execution of Javascrīpt.
可以用Javascrīpt来代替header。另外需要注意，采用这种方法需要浏览器支持Javascrīpt.

3b. Use output buffering (用输出缓存来解决):

... HTML codes ...
... 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 Javascrīpt since Javascrīpt method assumes the browser has Javascrīpt 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. Javascrīpt solution would be better if you know for sure your user has Javascrīpt turn on on their browser.

就像上面的代码那样，这种方法在生成页面的时候缓存，这样就允许在输出head之后再输出header了。本站的许愿板就是采用这种方法解决的header问题。

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 scrīpts 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。