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解决PHP中Warning: Cannot modify header information - headers already sent by的方法(转)

分类:PHP

今天在写升级版的secondary的时候发现了这个问题,将header()函数放置到<html>之后就会出现Warning: Cannot modify header information - headers already sent by...的警告,在网上找了相关资料,大概了解了下,特和朋友们分享。

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.

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。我就是采用这种方法解决的header问题。

如果在wordpress中出现该问题应该如何解决?字需要在主题文件functions.php中加入ob_start();即可,因为在wordpress机制中,functions.php先于header.php加载,因此functions.php中的内容是运行在<html>之前的,而当我们在调用函数的时候,如果函数中使用到了header()就会出现标题的情况,因此你需要仔细检查。

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。

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