#...

$searchstring = "";

switch($searchtype){
 case 'podgrupa_lijekova':
 case 'vrsta_lijekova':
 case 'itd':
  $searchstring = "AND $searchtype = $searchterm";
  break;
 case 'ime_lijeka':
 case 'itd':
  $searchstring = "AND $searchtype LIKE %$searchterm%";
}

$query .= $searchstring;
$result = $db->query($query);

#...