Try:
<?php
$commission = new WA_MySQLi_RS("commission",$horizon_i,0);
$commission->setQuery("SELECT Commissions.*, Request.TravelExpert FROM Request JOIN Commissions ON Request.ID=Commissions.RequestID WHERE Request.TravelExpert LIKE ? AND Commissions.Week IN ?");
$commission->bindParam("c", "".$_GET['expert'] ."", "-1"); //colname
$commission->bindParam("sl", "".implode(",",$_GET['Week']) ."", "-1"); //colname1
$commission->execute();
?>