这个想法是为每个删除按钮创建一个单独的表单，所以quoteid的正确值是send

The idea is create a separate form for each delete button so the right value for quoteid will be send

<?php
    require_once('config.php');
    $conn = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_DATABASE) 
    or die ('Cannot connect to db');
    $result = $conn->query("SELECT quoteid, quote, cat.catid catid, cat.name cat, scat.catid catid, scat.name scat, author FROM quotes q
                           INNER JOIN category cat ON q.catid = cat.catid LEFT OUTER JOIN category scat ON q.subcatid = scat.catid order by quoteid");
?>
    <table border='1px solid black'>
        <tr>
            <td>
                <b>Category</b>
            </td>
            <td>
                <b>Sub-Category</b>
            </td>
            <td>
                <b>Quote</b>
            </td>
            <td>
                <b>Author</b>
            </td>
        </tr>

        <?php
        while ($row = $result->fetch_assoc()) {
            ?>
            <tr>
                <td>
                    <?php echo $row['cat']; ?>
                </td>
                <td>
                    <?php echo $row['scat']; ?>
                </td>
                <td>
                    <?php echo $row['quote']; ?>
                </td>
                <td>
                    <?php echo $row['author']; ?>
                </td>
                <td>
                    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method='post' style="display: inline;">
                        <input type='hidden' id='quoteid' name='quoteid' value="<?php echo $row['quoteid']; ?>" />
                        <input type='submit' name='formDelete' id='formDelete' value='Delete' />
                    </form>
                </td>
            </tr>
            <?php
        }
        ?>
    </table>