About the PHP client and server-side doubts, how to deal with

Doubts about the PHP client and server side
I understand that the client is the browser execution, the server side is the part that the server executes.
I am puzzled is I write code when how to determine which code is executed by the browser side or server-side execution??

is not $_get, $_post, $REQUEST that uppercase variables are server-side?

Also, for example, I asked a question before, that is, on the form of duplicate submissions, CSDN friends say in the client write a judgment code, and then the server side write a judgment code. Although it has been basically solved, but I feel I must have written the client, did not understand the server side is going on.

I do not have PHP tutorial, please friends to say the details, thank you!



------Solution--------------------
Very simple, the code inside the tag is executed by the server, and the rest of the code is interpreted by the browser.
------Solution--------------------
PHP code is executed on the server side.
JS code is executed in the browser.

$_get...$_post are submitted by the client to the server side, of course, the server side to receive the line.

Server side, which is your PHP code section, the Post data receive processing Section

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