To solve your "problem" I recommend that you save this variable in a session, or even in a cookie to be able to use it later.
If you wanted to save in a session, on the page that takes the data from $_POST, you could enter this code:
<?php
session_start();//Colocar no inicio do código, se já não houver em alguma página que da include nessa
//resto do código
$id = $_POST['form'];
$_SESSION['dados'] = $id;
//resto do código
?>
Now to recover that data on any other page, you do the following:
<?php
session_start();//Colocar no inicio do código, se já não houver em alguma página que da include nessa
//resto do código
$id = $_SESSION['dados'];
?>
Remembering that by being SESSION, as soon as the browser is closed, this data will be "removed" from SESSION.
If you want to know about Cookies, this that page.
You can store using
sessionin that way$_SESSION['a'] = $ID– Rafael Augusto