http header前为什么不能有空格

<?php
session_start();
?>

在这个代码前如果有一个空格。When you make this tiny change, you are given a particularly venomous error
message:
Warning: session_start() [function.session-start]: Cannot send session
cookie - headers already sent by (output started at
/opt/lampp/htdocs/sites/startingchapter/sessions.php:1) in
/opt/lampp/htdocs/sites/startingchapter/sessions.php on line 3
The reason it is so important to not have anything before session_start() is
that the sessions framework makes use of the HTTP headers that form the mechanics
of the Web page. These special headers are pre-pended to each Web page and as
such, if you add any content before session_start(), the script will be trying to
send out content (such as the white space), then the headers, and then the main
content. This is not the way the Web works and, hence, PHP will shout at you in the
form of the previous warning message.

All About Headers
Every Web page on the Internet has some information called headers that
is invisible to most users. Headers store a number of pieces of information
that browsers care about. Here are some example headers:

GET / HTTP/1.
Host: www.yourfavewebsite.org
Connection: close
Accept-Encoding: gzip
Accept: text/xml,application/xml,application/xhtml+xml,
text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-gb,en;q=0.5
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-GB;
rv:1.8.0.3) Gecko/20060523 Ubuntu/dapper Firefox/1.5.0.3
Referer: http://www.someotherwebsite.net/

Beneath the headers lies the content. When you add content to the page,
the headers are added automatically. As such, when data is added to the
page, the headers are also sent and cannot be modified after they have
been sent. 一旦发送内容是heaer也跟着发送就不能更改了。
When you use sessions, the sessions system modifies some of these headers,
and this is why you must add session_start() before any data is
added to the page.