Your $_POST array will not be accessible on all pages. As much as I have understood your problem , You are trying to access $_POST array on some other file , and this can only be used in this file or the file you post to. You need to use SESSION in order to use these values
After POST Assign your values like this
$_SESSION["Category"] = $_POST['Category'];
Then on any other page just use
session_start();
on the top of page and then print your variable like
echo "Selected Category is " . $_SESSION["Category"] . ".<br>";