Warning: Cannot modify header information

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                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 scrīpt.
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 Javascrīpt (用Javascrīpt来解决):
? echo " 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):
?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 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。
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。  


本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u3/94426/showart_1900443.html