PHP contact form goes to a blank page

Question

My beloved PHP contact form goes to a blank page, email is not going through either. I've added an echo function at the end to see if it goes through the entire code. That worked, the echoed line was showing up. Quite a newbie in PHP, so please don't bite my head off. This is the project I'm working on: http://www.designbynoemi.co.uk.

Any direction would be highly appreciated.

And here is my code:

    <?php
/*
* Contact Form Class
*/

error_reporting(E_ALL); ini_set('display_errors', 1);

header('Cache-Control: no-cache, must-revalidate');
header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
header('Content-type: application/json');

$admin_email = '[email protected]'; // 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();

echo "Hello world!"; 

?>