<select class="form-control" name="kgrubu" id="kgrubu">
<option value="0">Kan Grubu Seçiniz</option>
<?php
    $query = $db->query("SELECT * FROM kgrubu", PDO::FETCH_ASSOC);
    if ( $query->rowCount() ){
        foreach( $query as $row ){
            echo '<option value="'.$row["id"].'" '.($row['id']==$user["kangrubu"]?'selected':null).'>'.$row["kagrubu"].'</option>';
        }
    }
?>
</select>