What is the meaning of the common method for php to obtain ip addresses?

What do php commonly used methods for obtaining ip addresses mean? Problem 1: The common method to obtain ip addresses is to use the "HTTP_CLIENT_IP", "HTTP_X_FORWARDED_FOR", and "REMOTE_ADDR" elements in the $ _ SERVER array. What are the values of these three elements? What do I mean by common methods for getting ip addresses in php?
Question 1:
You can use the "HTTP_CLIENT_IP", "HTTP_X_FORWARDED_FOR", and "REMOTE_ADDR" elements of the $ _ SERVER array to obtain the ip address.
What are the values of these three elements?
I checked on the Internet:
HTTP_CLIENT_IP: indicates the client ip address.
HTTP_X_FORWARDED_FOR: ip address of the client accessing the Internet through proxy
REMOTE_ADDR: connection ip address
My understanding is:
(1) if the client is directly connected to the Internet:
HTTP_CLIENT_IP: displays the client ip address.
HTTP_X_FORWARDED_FOR: no
REMOTE_ADDR: displays the client ip address.
(2) if the client uses the proxy server:
HTTP_CLIENT_IP: no
HTTP_X_FORWARDED_FOR: displays the client ip address.
REMOTE_ADDR: displays the proxy server ip address

Is that true?


Question 2:
The client's ip information is sent through the header? Why can't I see the relevant information when I use firefox to check the header?


Thank you!

------ Solution --------------------
1. you should be right. there is a classic if statement on the internet. it can be used if necessary. However, it can only identify the gentleman and is ineffective for the villain.
2, HTTP_X_FORWARDED_FOR, this should be available. The other two REMOTE_ADDR and CLIENT_IP personally think the same thing. they are all the IP addresses obtained by accept, but they are explained based on whether there is HTTP_XFORWARDED_FOR.
------ Solution --------------------
Only REMOTE_ADDR reflects the real public network endpoint of the user.
Others are additional content of various network devices for reference only
Because they can be constructed at will.