How to pass a variable by get method in self page?

What specifically is your question. How to pass an url parameter?

no! I use header(“location:/index?type=true”) but it doesn’t appear in my url. it doesn’t compile header function.

Have you seen the documentation? https://www.php.net/manual/en/function.header.php

sure!!! I used it before, in a php code. but in wordpress it doesn’t work.
My Main problem:
I have a form in a custom template and when it submitted, I want to show to user : ‘Your request submited’.

So can you just use the “action” in the form element?

No, I just return a true value to my form to show to user, any way

Can you pastebin all relevant code?
https://pastebin.com

summary of my code:
here

You can’t send headers or or sessions after your output to the browser. You need to either use an output buffer, or put that code at the top of the page before you output anything to the browser.

tnx Christian1012, Done!