电子邮件功能无法正确执行

好的，所以我为此工作了一个小时左右，我只是对问题是什么视而不见。有人可以指出我需要更改什么才能进行以下代码设置，以便在表单中输入的电子邮件地址向我服务器上的管理员电子邮件帐户发送电子邮件，让我们称之为 admin@email.com。下面是 html 形式和 php 代码。.PHP：

<?php
if(isset($_POST['email'])) {
// EDIT THE 2 LINES BELOW AS REQUIRED
$email_to = "admin@email.com";
$email_subject = "Your email subject line";
function died($error) {
    // your error code can go here
    echo "We are very sorry, but there were error(s) found with the form you submitted. ";
    echo "These errors appear below.<br /><br />";
    echo $error."<br /><br />";
    echo "Please go back and fix these errors.<br /><br />";
    die();
}
// validation expected data exists
if(!isset($_POST['first_name']) ||
    !isset($_POST['last_name']) ||
    !isset($_POST['email']) ||
    !isset($_POST['telephone']) ||
    !isset($_POST['comments'])) {
    died('We are sorry, but there appears to be a problem with the form you submitted.');       
}
$first_name = $_POST['first_name']; // required
$last_name = $_POST['last_name']; // required
$email_from = $_POST['email']; // required
$telephone = $_POST['telephone']; // not required
$comments = $_POST['comments']; // required
$error_message = "";
$email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+'.[A-Za-z]{2,4}$/';
if(!preg_match($email_exp,$email_from)) {
    $error_message .= 'The Email Address you entered does not appear to be valid.<br />';
}
$string_exp = "/^[A-Za-z .'-]+$/";
if(!preg_match($string_exp,$first_name)) {
    $error_message .= 'The First Name you entered does not appear to be  valid.<br />';
}
if(!preg_match($string_exp,$last_name)) {
    $error_message .= 'The Last Name you entered does not appear to be valid.<br />';
}
if(strlen($comments) < 2) {
    $error_message .= 'The Comments you entered do not appear to be valid.<br />';
}
if(strlen($error_message) > 0) {
    died($error_message);
}
$email_message = "Form details below.'n'n";
function clean_string($string) {
    $bad = array("content-type","bcc:","to:","cc:","href");
    return str_replace($bad,"",$string);
}
$email_message .= "First Name: ".clean_string($first_name)."'n";
$email_message .= "Last Name: ".clean_string($last_name)."'n";
$email_message .= "Email: ".clean_string($email_from)."'n";
$email_message .= "Telephone: ".clean_string($telephone)."'n";
$email_message .= "Comments: ".clean_string($comments)."'n";
// create email headers
$headers = 'From: '.$email_from."'r'n".
    'Reply-To: '.$email_from."'r'n" .
    'X-Mailer: PHP/' . phpversion();
    @mail($email_to, $email_subject, $email_message, $headers);  
    ?>
<!-- include your own success html here -->
Thank you for contacting us. We will be in touch with you very soon.
<?php
}
?>

.HTML：

<form name="contactform" method="post" action="send_form_email.php">
    <table width="450px">
        <tr>
            <td valign="top">
                <label for="first_name">First Name *</label>
            </td>
            <td valign="top">
                <input  type="text" name="first_name" maxlength="50" size="30">
            </td>
        </tr>
        <tr>
            <td valign="top"">
                <label for="last_name">Last Name *</label>
            </td>
            <td valign="top">
                <input  type="text" name="last_name" maxlength="50" size="30">
            </td>
        </tr>
        <tr>
            <td valign="top">
                <label for="email">Email Address *</label>
            </td>
            <td valign="top">
                <input  type="text" name="email" maxlength="80" size="30">
            </td>
        </tr>
        <tr>
            <td valign="top">
                <label for="telephone">Telephone Number</label>
            </td>
            <td valign="top">
                <input  type="text" name="telephone" maxlength="30" size="30">
            </td>
        </tr>
        <tr>
            <td valign="top">
                <label for="comments">Comments *</label>
            </td>
            <td valign="top">
                <textarea  name="comments" maxlength="1000" cols="25" rows="6">  </textarea>
            </td>
        </tr>
        <tr>
            <td colspan="2" style="text-align:center">
                <input type="submit" value="Submit">   <a   href="http://www.freecontactform.com/email_form.php">Email Form</a>
            </td>
        </tr>
    </table>
</form>