He did not forget an echo the '=' equals "echo" in this case ..
the problem is a little bit more complex , what you are trying to do is basically starting a second php instance within an existing one (<? ... <?)
so change this code:
Code:
<a href="<?=$_GET["categ"]?>_next.php\">
to
Code:
<a href=\"" . $_GET["categ"] . "_next.php\">
the two quotation marks after the href are very important btw ..