PHP电子邮件脚本响应消息显示html标记

我有一个PHP脚本，可以从html联系人表单发送电子邮件。电子邮件发送没有错误，但写入网页的响应消息包含html以及消息：

｛"status"：1，"html"："谢谢<p> "｝

如有任何帮助，我们将不胜感激。这是脚本：

<?php
/*
* Contact Form Class
*/

header('Cache-Control: no-cache, must-revalidate');
header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
header('Content-type: application/json');
$admin_email = 'fred.smith@ntlworld.com'; // Your Email
$message_min_length = 5; // Min Message Length

class Contact_Form{
    function __construct($details, $email_admin, $message_min_length){
        $this->name = stripslashes($details['name']);
        $this->email = trim($details['email']);
        $this->subject = 'Contact from Your Website'; // Subject 
        $this->message = stripslashes($details['message']);
        $this->email_admin = $email_admin;
        $this->message_min_length = $message_min_length;
        $this->response_status = 1;
        $this->response_html = '';
    }

    private function validateEmail(){
        $regex = '/^(['w-]+(?:'.['w-]+)*)@((?:['w-]+'.)*'w['w-]{0,66})'.([a-z]{2,6}(?:'.[a-z]{2})?)$/i';
        if($this->email == '') { 
            return false;
        } else {
            $string = preg_replace($regex, '', $this->email);
        }
        return empty($string) ? true : false;
    }

    private function validateFields(){
        // Check name
        if(!$this->name)
        {
            $this->response_html .= '<p>Please enter your name</p>';
            $this->response_status = 0;
        }
        // Check email
        if(!$this->email)
        {
            $this->response_html .= '<p>Please enter an e-mail address</p>';
            $this->response_status = 0;
        }
        // Check valid email
        if($this->email && !$this->validateEmail())
        {
            $this->response_html .= '<p>Please enter a valid e-mail address</p>';
            $this->response_status = 0;
        }
        // Check message length
        if(!$this->message || strlen($this->message) < $this->message_min_length)
        {
            $this->response_html .= '<p>Please enter your message. It should have at least '.$this->message_min_length.' characters</p>';
            $this->response_status = 0;
        }
    }

    private function sendEmail(){
        $mail = mail($this->email_admin, $this->subject, $this->message,
             "From: ".$this->name." <".$this->email.">'r'n"
            ."Reply-To: ".$this->email."'r'n"
        ."X-Mailer: PHP/" . phpversion());
        if($mail)
        {
            $this->response_status = 1;
            $this->response_html = "<p>Thank You!</p>";
        }
    }

    function sendRequest(){
        $this->validateFields();
        if($this->response_status)
        {
            $this->sendEmail();
        }
        $response = array();
        $response['status'] = $this->response_status;   
        $response['html'] = $this->response_html;
        echo json_encode($response);
    }
}

$contact_form = new Contact_Form($_POST, $admin_email, $message_min_length);
$contact_form->sendRequest();
?>