Explain PSR-7 like I'm five

You must comply with the rules your parents enforce, in your house, like

• say hello POST /path HTTP/1.1
• be polite 200 OK
• speak the same language as everyone (protocol)

The same rules must apply everywhere you go, including the school you like calling PHP.

house rules - The rules of the HTTP protocol like tools.ietf.org/html/rfc7230