The reason why there cannot be any output before header and session_start in php

The reason why there cannot be any output before header and session_start in php is: In the HTTP transmission text, 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 error-tolerant). For example, if it does not meet the format of "keyword: valuen", it will be directly wrong, but it will satisfy the "keyword" format. : valuen" format, will the client install the wrong understanding, or will it be understood correctly?

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