check php variable in javascript function

I've downloaded a template for web.

In this template, there is a php script to send email. In this script there is a validate() function, in which there is a variable $return_array. one of its value is $return_array['success'] that could be '0' or '1'. In another file, a javascript one, there is a click() function, that manages the value of 'success' in the php script doing if(html.success == '1')...but it does not work as expected, infact it is always '0'...what do I need to check?

Here is the html form:

<form method="POST" action="send_form_email.php" id="contactform">
            <div>
                <label for="name" style="color:white;">Inserisci il tuo nome</label>
                    <input type="text" class="input-field" id="name" name="name" value="">
            </div>
            <div>
                <label for="email" style="color:white;">Inserisci la tua e-mail</label>
                    <input type="text" class="input-field" id="email" name="email" value="">
            </div>
            <div>
                <label style="color:white;">Scrivi il tuo messaggio</label>
                    <textarea id="message" name="message" style="min-height: 160px;"></textarea>
            </div>                  
                <a id="button-send" href="#" title="Send Email" class="button" style="width:100%;">Invia E-Mail</a>
                <div id="success">Il tuo messaggio &egrave stato inviato correttamente!</div>
                <div id="error">Impossibile inviare il messaggio. Riprovare pi&ugrave tardi.</div>
        </form>

and here is the function into the php

<?php 
// EDIT THE 2 LINES BELOW AS REQUIRED
$send_email_to = "[email protected]";
$email_subject = "Feedback subject";
function send_email($name,$email,$email_message)
{
  global $send_email_to;
  global $email_subject;
  $headers = "MIME-Version: 1.0" . "\r\n";
  $headers .= "Content-type:text/html;charset=iso-8859-1" . "\r\n";
  $headers .= "From: ".$email. "\r\n";
  $message = "<strong>Email = </strong>".$email."<br>";
  $message .= "<strong>Name = </strong>".$name."<br>";  
  $message .= "<strong>Message = </strong>".$email_message."<br>";
  @mail($send_email_to, $email_subject, $message,$headers);
  return true;
}

function validate($name,$email,$message)
{
  $return_array = array();
  $return_array['success'] = '1';
  $return_array['name_msg'] = '';
  $return_array['email_msg'] = '';
  $return_array['message_msg'] = '';
  if($email == '')
  {
    $return_array['success'] = '0';
    $return_array['email_msg'] = 'email is required';
  }
  else
  {
    $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
    if(!preg_match($email_exp,$email)) {
      $return_array['success'] = '0';
      $return_array['email_msg'] = 'enter valid email.';  
    }
  }
  if($name == '')
  {
    $return_array['success'] = '0';
    $return_array['name_msg'] = 'name is required';
  }
  else
  {
    $string_exp = "/^[A-Za-z .'-]+$/";
    if (!preg_match($string_exp, $name)) {
      $return_array['success'] = '0';
      $return_array['name_msg'] = 'enter valid name.';
    }
  }

  if($message == '')
  {
    $return_array['success'] = '0';
    $return_array['message_msg'] = 'message is required';
  }
  else
  {
    if (strlen($message) < 2) {
      $return_array['success'] = '0';
      $return_array['message_msg'] = 'enter valid message.';
    }
  }
  return $return_array;
}

$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];


$return_array = validate($name,$email,$message);

if($return_array['success'] == '1')
{
    send_email($name,$email,$message);

}
header('Content-type: text/json');
echo json_encode($return_array);
die();
?>

and following the javascript code:

$('#button-send').click(function(event){
    $('#button-send').html('Invio in corso...');
    event.preventDefault();

    //$('html, body').scrollTo( $('#contact'), 'fast' );
    $.ajax({
        type: 'POST',
        url: 'send_form_email.php',
        data: $('#contactform').serialize(),
        success: function(html) {
            if(html.success == '1')
            {
                $('#button-send').html('Invia E-Mail');
                $('#success').show();

            }
            else
            {
                $('#button-send').html('Invia E-Mail');
                $('#error').show();
                console.log(html);
            }                   
        },
        error: function(){
            $('#button-send').html('Invia E-Mail');
            $('#error').show();
            console.log("not html.success");
        }
    });

});

EDIT:

I've edited the php part adding at the end the json_encode and content type, but when there is an error, like name missing or mail missing, I expect to see something appears near the input form, but it does not...