코어를 수정해야 한다는 게 무슨 말 인가요??어느 부분을 말씀 하시는 건지 제가 아직 잘 몰라서 이해를 못했습니다ㅠㅠㅠㅠ

list.skin.php

  
 <link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
    <script src="//code.jquery.com/ui/1.8.18/jquery-ui.min.js"></script>
    <script>
    $.datepicker.setDefaults({
        dateFormat: 'yy-mm',
        prevText: '이전 달',
        nextText: '다음 달',
        monthNames: ['1월', '2월', '3월', '4월', '5월', '6월', '7월', '8월', '9월', '10월', '11월', '12월'],
        monthNamesShort: ['1월', '2월', '3월', '4월', '5월', '6월', '7월', '8월', '9월', '10월', '11월', '12월'],
        dayNames: ['일', '월', '화', '수', '목', '금', '토'],
        dayNamesShort: ['일', '월', '화', '수', '목', '금', '토'],
        dayNamesMin: ['일', '월', '화', '수', '목', '금', '토'],
        showMonthAfterYear: true,
        yearSuffix: '년'
    });

        $(function() {
            $("#stx, #stx2, #e_Date").datepicker({
                dateFormat: 'yy-mm-dd'
            });
        });

    </script>
            <div style="width:<?php echo $width; ?>; text-align:left;">


       <!-- 날짜 검색 시작 { -->
            <form name="fsearch" method="get">
            <input type="hidden" name="bo_table" value="<?php echo $bo_table ?>">
            <input type="hidden" name="sca" value="<?php echo $sca ?>">
            <input type="hidden" name="sop" value="and">
            <input type="hidden" name="sfl" value="wr_12" />
            <tr><input type="hidden" name="sfl" value="wr_1"<?php echo get_selected($sfl, 'wr_1'); ?>>구매일자</input>
                <i class="fa fa-calendar" aria-hidden="true"></i> <input type="text" name="stx" value="<?php echo stripslashes($stx) ?>" required id="stx" class="sch_input" size="25" maxlength="20" placeholder="날짜검색">
                ~
                <i class="fa fa-calendar" aria-hidden="true"></i> <input type="text" name="stx2" value="<?php echo stripslashes($stx2) ?>" required id="stx2" class="sch_input" size="25" maxlength="20" placeholder="날짜검색">
            <input type="hidden" name="sfl" value="wr_subject"<?php echo get_selected($sfl, 'wr_subject'); ?>>약품명</input>
            <label for="stx3" class="sound_only">검색어<strong class="sound_only"> 필수</strong></label>        
                <input type="text" name="stx3" value="<?php echo stripslashes($stx3) ?>" required id="stx3" class="sch_input" size="25" maxlength="20" placeholder=" 검색어를 입력해주세요">               
            <button type="submit" value="검색" class="sch_btn"><i class="fa fa-search" aria-hidden="true"></i><span class="sound_only">검색</span></button>
            </tr>
            </form>
 </div>

common.lib.php

$str .= $op2;
            switch ($field[$k]) {
                 case "wr_12" :
 
                    $str .= " wr_1 between  '{$s[$i]}' and '$stx2' ";
 
                    break;
                case "mb_id" :
                case "wr_name" :
                    $str .= " $field[$k] = '$s[$i]' ";
                    break;

list.php

$is_search_bbs = false;
 
if ($sca || $stx || $stx === '0') {     //검색이면
    $is_search_bbs = true;      //검색구분변수 true 지정
    if($bo_table == "purchase_stock")
    $sql_search="{$sfl} between '{$stx}' and '{$stx2}'";
    else
    $sql_search = get_sql_search($sca, $sfl, $stx, $sop);
 
    // 가장 작은 번호를 얻어서 변수에 저장 (하단의 페이징에서 사용)

소스코드는 이렇습니다! list.skin.php에서
 <input type="hidden" name="stx" value="<?php echo $stx ?>">
    <input type="hidden" name="stx2" value="<?php echo $stx2 = $_GET['stx2'] ? $_GET['stx2'] : ''; ?>">
이렇게 수정한 부분도 있습니다!

여전히 sfl는 두 개가 있군요.
말씀드린 것부터 하나씩 수정해 보세요.

<form name="fsearch" method="get">
            <input type="hidden" name="bo_table" value="<?php echo $bo_table ?>">
            <input type="hidden" name="sca" value="<?php echo $sca ?>">
            <input type="hidden" name="sop" value="and">
            <input type="hidden" name="sfl2" value="wr_1"<?php echo get_selected($sfl2, 'wr_1'); ?>>구매일자</input>
                <i class="fa fa-calendar" aria-hidden="true"></i> <input type="date" name="stx" value="<?php echo stripslashes($stx) ?>" required id="stx" class="sch_input" size="25" maxlength="20" placeholder="날짜검색">
                ~
                <i class="fa fa-calendar" aria-hidden="true"></i> <input type="date" name="stx2" value="<?php echo stripslashes($stx2) ?>" required id="stx2" class="sch_input" size="25" maxlength="20" placeholder="날짜검색">
            <input type="hidden" name="sfl" value="wr_subject"<?php echo get_selected($sfl, 'wr_subject'); ?>>약품명</input>
            <label for="stx3" class="sound_only">검색어<strong class="sound_only"> 필수</strong></label>        
                <input type="text" name="stx3" value="<?php echo stripslashes($stx3) ?>" required id="stx3" class="sch_input" size="25" maxlength="20" placeholder=" 검색어를 입력해주세요">               
            <button type="submit" value="검색" class="sch_btn"><i class="fa fa-search" aria-hidden="true"></i><span class="sound_only">검색</span></button>
            </tr>
            </form>

이렇게 하시라는 말씀이 맞나요???