PHP中的复选框一直显示错误

Question

任何人都可以告诉我为什么无论什么时候提交，我的消息都会丢失？我很难编码，但这似乎很容易，也许我错过了一些明显的东西？

<?php
$name = $_POST['name']; //Grabs what is typed in the name box
$email = $_POST['email'];
$message = $_POST['message'];
$from = 'From: Rissotos'; //Will say where the email is coming from
$to = 'myemail@gmail.com'; //where the email is being sent
$subject = 'Rissotos Message';//Built in subject Line

 $body = "From: $name\n E-Mail: $email\n Message:\n $message";//Store all Variables
?> 

<?php
if ($_POST['submit'] && $human =='checked') {//checks spam 
if (mail ($to, $subject, $body, $from)) { 
    echo '<p>Your message has been sent!</p>';//Message if all is filled out 
} else { 
    echo '<p>Something went wrong, go back and try again!</p>';//Message if something was not filled out properly 
}
}else if ($_POST['submit'] && $human !='checked') {
echo '<p>No  Robots Allowed!</p>';
} 

?>

<form method="post" action="contactUs.php">

        <label>Name</label> 
        <input name="name" placeholder="Type Here">

        <label>Email</label>
        <input name="email" type="email" placeholder="Type Here">

        <label>Message</label>
        <textarea name="message" placeholder="Type Here"></textarea>

        <p> By checking this box, you validate that you are not a Robot.</p>
        <input name="human" type="checkbox" value="checked" />

        <input id="submit" name="submit" type="submit" value="Submit">

    </form>

php的第一部分仅用于构建电子邮件，我不认为这是问题所在。我不确定复选框如何命名＆＃34; human＆＃34;正在被调用或者如果＆＃34;被检查＆＃34;对于盒子来说价值就是问题。