HTML solves the problem of invalid table width setting

        <col style="width: 100px;"/>
        <col>
        <col style="width: 100px;"/>
        <col style="width: 100px;"/>
        <col style="width: 100px;"/>

.detail {
    padding-bottom: 50px;
    margin-top: 80px;
}

.detail_table {
    table-layout:fixed;/*The column width is set by the table width and column width. */
    margin: auto;/*table centered*/
    text-align: center;
    border: 1px solid #804040;
    border-collapse: collapse;
}

.detail_table th {
    padding: 26px;
}

.detail_table td {
    border: 1px solid #804040;
    padding-top: 16px;
    padding-bottom: 16px;
}

.special {
    text-align: left;
    padding-left: 20px;
}

<div class="detail">

    <table class="detail_table">

        <thead>
        <th colspan="5">Details of selected options</th>
        </thead>
        <!--If you write, it will follow the width you write, but if the width you write does not fill the table, it will be divided equally into each td according to the proportion of the size you give-->
        <!--Now the four columns are 100px, and the column without data takes up all the rest-->
        <col style="width: 100px;"/>
        <col>
        <col style="width: 100px;"/>
        <col style="width: 100px;"/>
        <col style="width: 100px;"/>

        <tbody>

        <tr>
            <td>
                Serial number
            <td>
                Symptoms
            <td>
                Conform to
            <td>
                Not compliant</td>
            <td>
                Unsure
        </tr>

        <?php $i = 1; ?>

        <?php foreach ($test as $item): ?>
            <!--Get the selection-->
            <?php $temp = $this->session->userdata('b' . $i) ?>

            <tr>

                <td>
                    <?php echo $i; ?>
                </td>

                <td>
                    <?php echo $item; ?>
                </td>

                <td>
                    <?php if ($temp == 1): ?>√<?php endif ?>
                </td>
                <td>
                    <?php if ($temp == 2): ?>√<?php endif ?>
                </td>

                <td>
                    <?php if ($temp == 3): ?>√<?php endif ?>
                </td>

            </tr>

            <?php $i++; ?>

        <?php endforeach; ?>

        </tbody>

    </table>

</div>