PHP告诉我填写所有字段

好的，所以我有一个联系表格：

<?php
$body = "From: $name'n E-Mail: $email'n Message:'n $message";
if ($_POST['submit']) {
    if ($name != '' && $email != '') {
        if ($human == '4') {                 
            if (mail ($to, $subject, $body, $from)) { 
                echo '<p>Your message has been sent!</p>';
            } else { 
                echo '<p>Something went wrong, go back and try again!</p>'; 
            } 
       } else if ($_POST['submit'] && $human != '4') {
           echo '<p>You answered the anti-spam question incorrectly!</p>';
       }
   } else {
       echo '<p>You need to fill in all required fields!</p>';
   }
}

?>

以及与之配套的 HTML 代码：

<form action="" method="POST"> 
<!--PHP Code above goes here-->
<div>
<label>Name</label>
    <br>
    <input name="name" placeholder="Type Here">
</div>
<div>   
    <label>Email</label>
    <br>
    <input name="email" type="email" placeholder="Type Here">
</div>
<div>
    <label>Message</label>
    <br>
    <textarea name="message" placeholder="Type Here"></textarea>
</div>
<div> 
    <label>*What is 2+2? (Spam protection)</label>
    <br>
    <input name="human" placeholder="Type Here">
</div>
<input id="submit" name="submit" type="submit" value="Submit">
</form>

但即使填写了所有字段，它也会一直告诉我填写所有必填字段。提前谢谢。