If you got this message: "Warning: Cannot modify header information - headers already sent by ...."
如果在執行php程序時看到這條警告:"Warning: Cannot modify header information - headers already sent by ...."
Few notes based on the following user posts:
有以下幾種解決方法:
1. Blank lines (空白行):
Make sure no blank line after <?php ... ?> of the calling php script.
檢查有<?php ... ?> 后面沒有空白行，特別是include或者require的文件。不少問題是這些空白行導致的。
2. Use exit statement (用exit來解決):
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來解決):
<? 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來代替header。但是上面的這段代碼我沒有執行成功... 另外需要注意，采用這種方法需要瀏覽器支持Javascript.
3b. Use output buffering (用輸出緩存來解決):
<?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.
就像上面的代碼那樣，這種方法在生成頁面的時候緩存，這樣就允許在輸出head之后再輸出header了。
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結果最后還是這個問題：
原來是php.ini里面的配置出了問題，output_buffering參數默認為off的，現在將它設為”on”就ＯＫ了。