錯誤提 示：Cannot modify header information - headers already sent by ....

昨晚在轉換編碼的時候，有一個 頁面需要在head中申明utf-8的編碼，但是這與程式中的一處header產生了衝突。google了一下，找到幾種解決方法，翻譯整理一下：

If you got this message: "Warning: Cannot modify header information - headers already sent by ...."

如果在執行php程式時看到這 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 scrīpt.

檢查有<?php ... ?> 後面沒有空白行，特別是include或者require的檔案。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後加上 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來解決):

<? echo "<scrīpt> self.location( file.php );</scrīpt>"; ?>

Since it s a scrīpt, it won t modify the header until execution of Javascrīpt.

可以用Javascrīpt來 代替header。另外需要注意，採用這種方法需要瀏覽器支援Javascrīpt.

3b. Use output buffering (用輸出快取來解決):---本人用這種方法解決的！已證實可以！

<?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.

就像上面的程式碼那樣，這種方法 在生成頁面的時候快取，這樣就允許在輸出head之後再輸出header了。本站的許願板就是採用這種方法解決的header問題。

4.set output_buffering = On in php.ini (開啟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.

這種方法和3b的方法理論上是一樣 的。但是這種方法開啟了所有php程式的輸出快取，這樣做可能影響php執行效率，這取決於伺服器的效能和程式碼的複雜度。