How to pass variable id through href in php

<table class="table table-hover">
    <thead>
        <tr>
            <th>Subject</th>
            <th>Add Topic</th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <?php foreach($subjects as $sub):?>
            <td><?php echo $sub->subject;?></td>
            <td><a href='approve.php?id=".$sub->id."' role="button" class="btn">Add Topic</a></td>
        </tr>
        <?php endforeach;?>
    </tbody>
</table>