Analysis of the reasons why there cannot be output before header and session_start in PHP_PHP Tutorial

In the HTTP transmission text, it is stipulated that the order of header and content must be: header first and content last, and the format of the header must meet the format of "keyword: valuen".

1. If there is output content before the header is output, it will cause a misunderstanding of the header (although it is now fault-tolerant). For example, if it does not meet the format of "keyword: valuen", it is fine, but it is directly wrong. , but after meeting the format of "keyword: valuen", does the client install the wrong understanding, or does it follow the correct understanding?

2. When the session is opened, it will implicitly trigger whether to use header ("Set-Cookie: sid=xxxxxx"), which is actually an implicit header call.