function getRandomCode($length = 8) {
            $characters = 'lera_aw_petana_dabne_ka_atawe_bbet_ba_kod';
            $charactersLength = strlen($characters);
            $randomString = '';
            for ($i = 0; $i < $length; $i++) {
                $randomString .= $characters[rand(0, $charactersLength - 1)];
            }
            return $randomString;
        }

 getRandomCode()

<form>
  First name:<br>
  <input type="text" name="firstname">
  <br>
  student code:<br>
  <input type="text" name="studentcode">
</form>