게시판 분류를 셀렉트 박스로 추출 방법

<?php if ($is_category) { ?>
    <form action="<?php echo $bo_table ?>" method="get">
        <label for="bo_cate_select"><?php echo $board['bo_subject'] ?> 카테고리</label>
        <select name="sca" id="bo_cate_select">
            <?php $selected_all = (!isset($_GET['sca']) || $_GET['sca'] === '') ? 'selected' : '';?>
            <option value="" <?php echo $selected_all ?>>전체</option>

            <?php foreach ($categories as $category) { ?>
                <?php $selected = (isset($_GET['sca']) && $_GET['sca'] == $category) ? 'selected' : '';?>
                <option value="<?php echo htmlspecialchars($category); ?>" <?php echo $selected ?>><?php echo htmlspecialchars($category); ?></option>
            <?php } ?>
        </select>
        <input type="submit" value="선택">
    </form>

<?php } ?>