在做留言版时,提交留言后需要验证码。但这个验证码是从哪来的
<div class="col-lg-12" data-aos="fade-up" data-aos-delay="300">
<form method='post' class="php-email-form" id="form_do" name="form_do" action="<yang:gbookaddurl />">
<div class="form-row">
<div class="col-lg-6 form-group">
<input type="text" name="name" class="form-control" id="inputUsername" placeholder="Your Name" data-rule="minlen:4" data-msg="Please enter at least 4 chars" />
<div class="validate"></div>
</div>
<div class="col-lg-6 form-group">
<input type="email" class="form-control" name="email" id="inputEmail" placeholder="Your Email" data-rule="email" data-msg="Please enter a valid email" />
<div class="validate"></div>
</div>
</div>
<div class="form-group">
<textarea id="inputContent" class="form-control" name="message" rows="5" data-rule="required" data-msg="Please write something for us" placeholder="primary coverage"></textarea>
<div class="validate"></div>
</div>
<div class="mb-3">
<div class="loading">Loading</div>
<div class="error-message"></div>
<div class="sent-message"></div>
</div>
<div class="text-center">
<button type="submit" id="submit">Send Message</button>
</div>
</form>
</div>