session_start()在发送输出后工作 [英] session_start() works after output being sent

问题描述

我刚刚发现了一些奇怪的东西.我认为，正如PHP手册所述，在将任何输出发送到浏览器之前，必须先调用session_start():

I just noticed something strange. I thought that, as PHP's manual says, session_start() must be called before any output is sent to the browser:

要使用基于cookie的会话，必须在输出任何内容到浏览器之前调用 session_start().

因此，出于好奇，我创建了两个脚本.一个是 write.php :

So, just for curiosity, I've created two scripts. One is write.php:

<?php
echo 'foo';

session_start();
$_SESSION['bar'] = 'baz';
?>

另一个是 read.php :

<?php
echo 'foo';

session_start();
var_dump($_SESSION['bar']);
?>

令人惊讶的是，甚至在echo foo 之后，会话仍被写入和读取.

And surprisingly, the session is written and read even after echoing foo.

但是，如果我在echo之后添加对flush()的调用，Apache的错误日志将报告:

However, if I add a call to flush() after the echos, Apache's error log reports:

[2012年1月3日星期二11:57:21] [错误] [客户端127.0.0.1] PHP警告:session_start():无法发送会话缓存限制器-标头已在/var/www/sessions/write.php中发送在第5行 [2012年1月3日星期二11:57:21] [错误] [客户端127.0.0.1] PHP堆栈跟踪: [2012年1月3日星期二11:57:21] [错误] [客户端127.0.0.1] PHP 1. {main}()/var/www/sessions/write.php:0 [2012年1月3日星期二11:57:21] [错误] [客户端127.0.0.1] PHP2.session_start()/var/www/sessions/write.php:5

[Tue Jan 03 11:57:21 2012] [error] [client 127.0.0.1] PHP Warning: session_start(): Cannot send session cache limiter - headers already sent in /var/www/sessions/write.php on line 5 [Tue Jan 03 11:57:21 2012] [error] [client 127.0.0.1] PHP Stack trace: [Tue Jan 03 11:57:21 2012] [error] [client 127.0.0.1] PHP 1. {main}() /var/www/sessions/write.php:0 [Tue Jan 03 11:57:21 2012] [error] [client 127.0.0.1] PHP 2. session_start() /var/www/sessions/write.php:5

所以，我的问题是:为什么在echo处理完某些东西后正确编写了会话?它不是立即发送到浏览器吗?而且，如果是这样，是否意味着我可以在任何地方开始会话，只要我之前不致电flush()?

So, my questions is: why is the session written correctly after echoing something? Isn't it immediately sent to the browser? And, if so, does it mean I can start the session anywhere, as long as I don't call flush() before?

推荐答案

要使用基于cookie的会话，必须在调用session_start()之前 将任何内容输出到浏览器.

To use cookie-based sessions, session_start() must be called before outputing anything to the browser.

是的.服务器端cookie设置(与JavaScript cookie设置不同)通过发送HTTP标头来工作. HTTP标头位于实际文档之前:一旦开始发送文档，就没有地方放置其他标头了.

That's true. Server-side side cookie setting (unlike JavaScript cookie setting) works by sending an HTTP header. HTTP headers go before the actual document: once you start sending the document, there's no place for further headers.

在您的情况下，将发生以下情况:

In your case, what happens is that this line:

echo 'foo';

...实际上并不将输出发送到浏览器.而是将一些输出添加到队列中，稍后再发送.将PHP解释器配置为保留此输出，直到发生特定事件为止(可能脚本结束或队列达到特定大小).

... does not actually send output to the browser. Instead, it adds some output to a queue that will be sent later. The PHP interpreter is configured to hold this output until certain event happens (possibly, the scripts ends or the queue reaches certain size).

output_buffering 指令很可能是可疑的.

The output_buffering directive is the likely suspect.

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