What should I do if I have doubts about the php client and server?

Regarding questions about the php client and the server, I understand that the client is executed by the browser, and the server is executed by the server. I am confused. how can I determine whether the code is executed by the browser or by the server ?? Is it because $ _ GET, $ _ POST, $ requests and other uppercase variables are server-side variables? Also, for example, I used to ask a question about the form's questions about the php client and the server.
I understand that the client is executed by the browser, and the server is executed by the server.
I am confused. how can I determine whether the code is executed by the browser or by the server ??

Is it because $ _ GET, $ _ POST, $ requests and other uppercase variables are server-side variables?

Also, for example, I have asked a question before, that is, about repeated form submission. on CSDN, friends say that they should write a judgment code on the client side, and then write a judgment code on the server side. Although it has been basically solved, I feel that I have only written the client, but I have not figured out what is going on the server.

I don't have any php tutorials. please give me more details. thank you!



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Very simple, The code in the tag is executed by the server, and other codes are interpreted by the browser.
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PHP code is executed on the server.
JavaScript code is executed in a browser.

$ _ GET... $ _ POST is submitted by the client to the server. of course, the server must receive the request.

The server, that is, your php code, and POST data receiving and processing