If you are sure, that the array is containing integers, why don't you do it like this:

$array=array(1,2,3);
if (sizeof($array) > 0 {
  $sql = "UPDATE mytable SET val_to_update = XX WHERE id IN(".implode(',', $array).")";
  mysql_query($sql);
}

If you are sure, that the array is containing integers, why don't you do it like this:

$array=array(1,2,3);
if (sizeof($array) > 0 {
  $sql = "UPDATE mytable SET val_to_update = XX WHERE id IN(".implode(',', $array).")";
  mysql_query($sql);
}

If you want to use prepared statement you could create your sql using this code:

$array=array(1,2,3);
    if (sizeof($array) > 0 {
       $placeholders = array();
       for($i=0; $i<sizeof($array); $i++) {
         $placeholders[] = '?';
       }
      $sql = "UPDATE mytable SET val_to_update = XX WHERE id IN(".implode(',', $placeholders).")";
  // .....
}

If the values in the $array exists in another table you could use something like this:

$sql = "UPDATE mytable SET val_to_update = XX WHERE id IN (SELECT id FROM another_table WHERE condition = 1)";