PHP: $_GET id from mysql, loop result, and print urls with id:s

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I am creating blog for practice and i'm stuck with creating category URLs. I would like to generate URLs like www.page.com?id= automatically, when i loop data from mysql. After that when i click those links, it will print blog posts by category id.My code so far:\[code\] <?php $categoryid = intval($_GET['category_id']); $idsql = "SELECT * FROM Posts WHERE category_id ='$categoryid'"; $query = mysql_query($idsql) or die (mysql_error()); while ($array = mysql_fetch_array($query)) { //URLs? } ?>\[/code\]By the way now it's printing: "Notice: Undefined index: category_id.....", so what should i do with that? Thanks!