Need help with AJAX contact form

<!-- Email -->
         <article id="email" class="panel">

            <header>
              <h2>Email Me</h2>
            </header>
            <form action="send.php" method="post">
              <div id="contactForm">
                <div class="row half">
                  <div class="6u">
                    <input type="text" class="text" name="name" placeholder="Name" />
                    <!--Error message-->
                    <span class="error" id="err-name">please enter  your name</span>
                    <!--End error message-->
                  </div>
                  <div class="6u">
                    <input type="text" class="text" name="email" placeholder="Email" />
                    <!--Error message-->
                    <span class="error" id="err-email">please enter your e-mail</span>
                    <span class="error" id="err-emailvld">e-mail is not a valid format</span>
                    <!--End error message-->
                  </div>
                </div>
                <div class="row half">
                  <div class="12u">
                    <input type="text" class="text" name="subject" placeholder="Subject" />
                  </div>
                </div>
                <div class="row half">
                  <div class="12u">
                    <textarea name="message" placeholder="Message"></textarea>
                    <!--Error message-->
                    <span class="error" id="err-message">please enter your message</span>
                   <!--End error message-->
                  </div>
                </div>
                <div class="row">
                  <div class="12u">
                    <input type="submit" class="button" value="Send Message" id="submit_btn" />
                    <div id="success"><h2>Your message has been sent. Thank you!</h2></div>
                  </div>
                </div>
              </div>
            </form>
          </article>

      </div>

<?php
$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];

//Validate first
if(empty($name)||empty($email)||empty($message)) 
{
echo "Name and email and message are required!";
header('Location: index.html');
}
//validate against any email injection attempts
if(IsInjected($email))
{
echo "Bad email value!";
header('Location: index.html');
}


$msg =  " Name : $name \r\n"; 
$msg .= " Email: $email \r\n";
$msg .= " Message : ".stripslashes($_POST['message'])."\r\n\n";
$msg .= "User information \r\n"; 
$msg .= "User IP : ".$_SERVER["REMOTE_ADDR"]."\r\n"; 
$msg .= "Browser info : ".$_SERVER["HTTP_USER_AGENT"]."\r\n"; 
$msg .= "User come from : ".$_SERVER["SERVER_NAME"];

$recipient = "ashleynschneider29@gmail.com";  
$subject =  "Sender information";
$mailheaders = "From: $email\r\nReply-To: $email\r\nReturn-Path: $email\r\n";
$ok = mail($recipient, $subject, $msg, $mailheaders);

 if(isset($ok)){
header('Location: index.html'); 
 }else if(! isset($ok)){
echo "Name and email are required !";
 header('Location: index.html');
}


// Function to validate against any email injection attempts
function IsInjected($str)
{
  $injections = array('(\n+)',
          '(\r+)',
          '(\t+)',
          '(%0A+)',
          '(%0D+)',
          '(%08+)',
          '(%09+)'
          );
  $inject = join('|', $injections);
  $inject = "/$inject/i";
  if(preg_match($inject,$str))
{
    return true;
 }
  else
{
    return false;
 }
}
?>

/*click handler on the submit button*/
    $('#submit').click(function(){ 
        $('.error').fadeOut('slow'); 

        var error = false; 
        var name = $('input#name').val(); 
        if(name == "" || name == " ") {
            $('#err-name').fadeIn('slow'); 
            error = true; 
        }


            var msg = $('textarea#message').val(); 
            if(msg == "" || msg == " ") {
                $('#err-message').fadeIn('slow'); 
                error = true; 
            }

        var email_compare = /^([a-z0-9_.-]+)@([da-z.-]+).([a-z.]{2,6})$/; 
        var email = $('input#email').val(); 
        if (email == "" || email == " ") { 
            $('#err-email').fadeIn('slow'); 
            error = true;
        }else if (!email_compare.test(email)) { 
            $('#err-emailvld').fadeIn('slow'); 
            error = true;
        } 

        if(error == true) {
            return false;
        }

        var data_string = $('.contactForm').serialize(); 


        $.ajax({
            type: "POST",
            url: $('.contactForm').attr('action'),
            data: data_string,
            timeout: 6000,
            error: function(request,error) {
                if (error == "timeout") {
                    $('#err-timedout').fadeIn('slow');
                }
                else {
                    $('#err-state').fadeIn('slow');
                    $("#err-state").html('An error occurred: ' + error + '');
                }
            },
            success: function() {
                    $('#success').fadeIn('slow');
                        }
        });

        return false; 
    });