A GET method is used for name value pairs in a url.
Example: www.test.com?var1=var1&var2=var2
Here if we use $_GET['var1']
the expected value would be var1
.
Just change your links to reflect the desired variable
href="forumdocument.php?rowid=".$row['ID']."
Then you can use retrieve the id
using something like this on forumdocument.php
$rowid = $_GET['rowid'];