问PHP联系人页-如何指定输入字段的最少字母

EN

<?php

if ($_POST['submit']) {

  if (!$_POST['firstname']) {
    $error="<br/>- Please enter your first name";
  }
  
  if (!$_POST['lastname']) {
    $error="<br/>- Please enter your last name";
  }

  if (!$_POST['email']) {
    $error.="<br/>- Please enter your email";
  }
  if (!$_POST['message']) {
    $error.="<br/>- Please enter a message";
  }
  if (!$_POST['check']) {
    $error.="<br/>- Please confirm you are human";
  }
  
  if ($error) {
    $result='<div class="alert alert-danger" role="alert">
    <strong>Whoops, there is an error</strong>. Please correct the following: '.$error.'</div>';
  } else {
    mail("myname@mywebsite.com", 
   "Contact message", 
    "Name: ".$_POST['name']."
    Email: ".$_POST['name']."
    Message: ".$_POST['message']);
    
    {
    $result='<div class="alert alert-success" role="alert">Thank you, I\'ll be in touch shortly. </div>';
    }
  }
}
?>

#HTML CODE#

  <form method="post" role="form">
            <div class="row">
            <div class="col-md-3"></div>
                  <div class="contact-icon">
                      <i class="fas fa-user"></i>
                  </div>

                  <div class="form-inline">
                    <div class="col-md-9 form-group">
                          <label for="name">your name</label>
                          <span class="fieldSpan">
                          <input type="text" 
                          name="name" 
                          id="input-field-name"
                          class="form-control" 
                          placeholder="your name" 
                          value="<?php echo $_POST['name']; ?>"> 
                    </div>
                </div>
            </div>

        <div class="row">
        <div class="col-md-3"></div>
              <div class="contact-icon">
                <i class="fas fa-envelope"></i>
              </div>

              <div class="form-inline">
              <div class="col-md-9 form-group">
                      <label for="email">your email address</label>
                      <span class="fieldSpan">
                      <input type="email" 
                      name="email" 
                      id="input-field-email"
                      class="form-control" 
                      placeholder="yourname@website.com" 
                      value="<?php echo $_POST['email']; ?>">
                  </div>
                </div>
         </div>

        <div class="row">
        <div class="col-md-3"></div>
              <div class="contact-icon">
                  <i class="fas fa-pencil-alt"></i>
              </div>
        
              <div class="form-inline">
                 <div class="col-md-9 form-group">
                        <label for="message">your message</label>
                        <span class="fieldSpan vertical-align:top">
                        <textarea name="message" 
                        rows="5" 
                        id="input-field-message"
                        class="form-control" 
                        placeholder="your message">
                        <?php echo $_POST['message']; ?></textarea>
                        <br>
                        <br>
                    </div>
                    </div>
              </div>
        </div>
 

              <div class="col-md-12">
                 <div class="checkbox">
                    <label>
                        <input type="checkbox" 
                        name="check"> 
                        Check this box if you're human.
                      </label>
                  </div>
              </div>
        </div>

      <div class="col-md-3"></div>
      <div class="row">
          <div class="col-md-9">
              <input type="submit" 
              name="submit" 
              class="btn btn-secondary" 
              value="send message"/>
            </div> 
        </div>

     </form>
      
     </div>
      </div>
    </div>
  </section>
  

 <!-- ----------- SCRIPTS------------ -->

    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>

<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>

<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>