"In" clause need values as '2','23','11','4' instead of "2,23,11,4,"
so manipulate your array for result.
$innval = '';
foreach($users as $user) {
if($user != '')
$innval .= "'".$user."',";
}
$innval = substr($innval,0,-1); // to remove last extra ,
and now use this $innval variable in your query to get result
$checkSQL = mysql_query("SELECT * FROM this WHERE x>'$y' && v<'$z' && user
IN ($innval)");
Hope this helps