PHP中header和session_start前不能有输出原因分析
在http传输文本中，规定必须 header和content顺序必须是：header在前content在后，并且header的格式必须满足“keyword: value\n”这种格式。
1、在header输出之前有输出内容的话，就会造成对header的错误理解（尽管现在已经能容错了），例如不是满足“keyword: value\n”的格式还好，直接错误了，但是满足“keyword: value\n”这个格式以后，客户端是否安装错误理解，还是按照正确理解？
2、session开启是会隐含的触发是否用header(“Set-Cookie: sid=xxxxxx”),也就是其实还是一个隐式的header调用
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