Php passing values through hyper link and use in next page

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PHP

Good Day

I have a question
i want to pass a value through hyper link to other page like
apply
and in next page i.e. somepage.php
i have a form that will use value which is sent.
$somevar = $_GET['id'];
now will input fields in this page which is a form.
issue is that when i submit the page the page gets refreshed and $somevar
gets empty when the form is submitted.

also i want to display the success message or error message after form is submiited
and then redirect to page from where link (id) was sent.
this is seniorio

1. main page contains no of jobs when i click apply link
apply
2. apply page is form page that inputs user data e.g.

form name="abc" action="result.php" method="POST"
name:
age:
address:
email: etc

/Form

3. result.php
here i perform validation then, save data in database
insert into tablename(fied1,field2...) values (val1,val2,...);
now i want to show success message using java script
echo "Application Submitted Successfully";
header("Location:mainpage.php");

Issue
issue is that record is saved but alert message is not displayed instead it goes to mainpage.php without showing any message;

What I have tried:

i tried many things but nothing happend