<select data-errormessage-value-missing="Course is required"
        class="form-control" 
        name="topic_id" id="topic_id"
        onchange="getOtherTopic(this.value);">
    <option value="">Select Topic</option>
    <?php foreach ($child_topic as $v_topic) { ?>
        <option value="<?php echo $v_topic->topic_id; ?>"><?php echo $v_topic->topic_name; ?></option>
    <?php } ?>
        <option value="other">other</option>
</select>

