php header函数的问题，比如说header之前不能有实际的输出
原文链接：PHP Header用于页面跳转要注意的几个问题总结php技巧脚本之家
搜到了这篇文章，开头就说了三个关于PHP header()的注意事项，
- location和“:”号间不能有空格，否则会出错。
- 在用header前不能有任何的输出。
- header后的PHP代码还会被执行。
第3点测试结果无误。
第1点，测试大概是指的 header('Location: http://segmentfault.com/'); 中 Location 和 : 不能有空格吧，但其实是可以有的。
主要是第2点，header前不能有任何的输出有疑问，我测试了下面的代码，能成功跳转啊：
<html>asdf</html>
<?php
header('Location : http://segmentfault.com/');
到底要怎样测试才能报错呢？php手册上的“Remember that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP”到底是啥意思？
回复内容：
原文链接：PHP Header用于页面跳转要注意的几个问题总结php技巧脚本之家
搜到了这篇文章，开头就说了三个关于PHP header()的注意事项，
- location和“:”号间不能有空格，否则会出错。
- 在用header前不能有任何的输出。
- header后的PHP代码还会被执行。
第3点测试结果无误。
第1点，测试大概是指的 header('Location: http://segmentfault.com/'); 中 Location 和 : 不能有空格吧，但其实是可以有的。
主要是第2点，header前不能有任何的输出有疑问，我测试了下面的代码，能成功跳转啊：
<html>asdf</html>
<?php
header('Location : http://segmentfault.com/');
到底要怎样测试才能报错呢？php手册上的“Remember that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP”到底是啥意思？
header函数内的String原则上会被直接加到HTTP头里面去。所以说，如果发送
Location : http://with.blank.com
这种并不符合HTTP规范的HTTP头，很可能整个HTTP Response都无法解析。
幸运的是，PHP貌似为你修复了这个HTTP头格式问题。
关于HTTP头先发的问题，的确不应该在HTTP内容输出之后输出HTTP头。但是服务器会缓存输出，虽然你使用了echo或者print，但在那个时刻服务器还没有将这些内容作为HTTP报文输出，这个时候你仍然有机会修改HTTP头。
如果你只是单纯想要引发一个错误的话，很简单，在header()之前输出一个绝对超过缓存大小的HTTP内容就可以了。以下是不良示范。
<?php
for($i = 0; $i < 1e5; $i++) echo $i;
header("ABCABC: BCDBCD");
@scaret 应该是对的, 其实就是缓存的问题.
<code><?php
echo "<html></html>";
ob_flush();
flush();
header('Location: http://segmentfault.com/');
</code>得到
<code>Warning: Cannot modify header information - headers already sent by (output started at ... </code>
你把html标签，换成PHP内部的echo，就会报错了。
php脚本中的html标签，其实是等整个PHP执行完，才会发送给用户的。
所以在你demo里面的header('Location : http://segmentfault.com/');前面，并没有任何输出。
header('Location : http://segmentfault.com/');这个方法，是在响应头里面，发一个302重定向。而响应头当然要比响应主体要提前输出。
详情可以用fiddler抓包看看
楼主你好，我和你有同样的疑问，我也是在header 200 之前进行了输出，按照二楼的方法，我甚至输出了：
<code>//不要轻易加入以下代码，可能会搞垮你的浏览器，只是为了说明 for($i = 0; $i < 1e100; $i++) echo $i; </code>
超过几十兆的数据，客户端浏览器依然没有报错，我猜测可能是web server端（例如apache或者nginx）已经对这种情况进行了优化，也许header之前不能有输出只是一个古老的建议，可能是因为php老版本存在的bug导致的，现在估计这个bug已经被修正，所以没有把报错。
另外有人说是因为没有马上进行客户端输出，所以我使用下面的代码：
<code>for($i = 0; $i < 1e5; $i++) echo $i;
ob_flush();
flush();
header('HTTP/1.1 200 OK');
</code>依然没有报错，谁能解答呢？
我只能说目前的php已经可以识别头输出，然后将头输出单独取出然后在response的时候优先输出，然后就成功避免了header之前有输出的问题。
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