<form action="" method="POST">
 Select Year:<select name="sortyear" id="sortyear">
                 <option value="2012">2012</option>
                 <option value="2013">2013</option>
                 <option value="2014">2014</option>
                 <option value="2015">2015</option>
             </select>
 Select Week:<select name="week" id="week">
          ...</select>

<?php $sql = mysql_query("SELECT SUM(credit) AS credit, depot, Date_Format(`timestamp`,'%U%Y') AS period FROM transactions where Date_Format(`timestamp`,'%U%Y')='102013' group by Date_Format(`timestamp`,'%U%Y') DESC"); while($row11 = mysql_fetch_array( $sql ))
   {
       $credit = $row11['credit'].'<br>';
       $depot = $row11['depot'].'<br>';
       $period = $row11['period'].'<br>';
   }

$sortyear = $_REQUEST['sortyear'];
$week = $_REQUEST['week'];?>

SELECT ...
...
WHERE (YEAR(timestamp) = 2013) AND (MONTH(timestamp) = 10)
GROUP BY YEAR(timestamp), MONTH(timestamp)