PHP-“标头已发送”错误取决于输出长度？

Question

I have a script which outputs mysql cell data. The "content" cell contains text output, which is of varied length.

When the contents of the "content" cell are small (lets say, a few lines of text), everything works fine. However, when the output reaches several paragraphs or more, I get the 'headers already sent' error.

Does it depend on the output length? Where can I read more about it? The answers I've found on SO mention nothing of such output length-dependency.

 44:   echo "
 45:       <p>".$article['content']."</p>
 46:   ";

If the size of the 'content' output is large, the script produces the following error:

PHP Warning: Cannot modify header information - headers already sent by (output started at /home/mantas/htdocs/asm/article.php:46) in /home/mantas/htdocs/asm/include/comments_class.php on line 56

Answer 1

PHP will buffer output if you want it to. You can control this programmatically with ob_start() , etc. However, there is a further option to set output buffering on in php.ini.

Setting output_buffering=on enables it, while setting output_buffering=4096 will set a limit to the buffer size. phpinfo() should tell you if this is enabled, and what the buffer size is.

The PHP reference is here

Answer 2

The "headers already send" warning means, you modify the http headers somewhere in your code after you send output to the Client(ie with echo , whitespaces, etc..).

This warning itself has nothing to do with the content length.

There are more methods, wich modify the headers:

• header / header_remove
• session_start / session_regenerate_id
• setcookie / setrawcookie

Answer 3

I also encountered "headers already sent by ..." problems using PHP's xmlWriter.

I had inserted the "header" instructions for managing the page output type at the end of the script:

I solved it by moving the instructions to the beginning of the script, this order used:

// init header earlier on script
header('Content-Type: text/xml; charset=utf-8');
header('Cache-Control: max-age=0');
header('Content-Disposition: inline');

// new xmlwriter object
$xml = new XMLWriter();

// set direct output stream
$xml->openURI('php://output');

// some code here for creating the xml output
// ....
// ....

// render xml to output
$xml->flush();