$('document').ready(function () {
    $('form').submit(function (e) {
        //prevent the form from submitting via the browser redirect
        e.preventDefault();
        //grab attributes and values out of the form
        var data = {
            email: $('#mc-email').val()
        };
        var endpoint = $(this).attr('action');
        //make the ajax request
        $.ajax({
            method: 'POST'
            , dataType: "json"
            , url: endpoint
            , data: data
        }).success(function (data) {
            if (data.id) {
                //successful adds will have an id attribute on the object
                alert('thanks for signing up');
            }
            else if (data.title == 'Member Exists') {
                //MC wil send back an error object with "Member Exists" as the title
                alert('thanks, but you are alredy signed up');
            }
            else {
                //something went wrong with the API call
                alert('oh no, there has been a problem');
            }
        }).error(function () {
            //the AJAX function returned a non-200, probably a server problem
            alert('oh no, there has been a problem');
        });
    });
});

<?php
//fill in these values for with your own information
$api_key = 'yes my apikey xxxx';
$datacenter = 'us14';
$list_id = 'listid i know';
$email = $_POST['email'];
$status = 'subscribed';
if(!empty($_POST['status'])){
    $status = $_POST['status'];
}
$url = 'https://'.$datacenter.'.api.mailchimp.com/3.0/lists/'.$list_id.'/members/';
$username = 'apikey';
$password = $api_key;
$data = array("email_address" => $email,"status" => $status);
$data_string = json_encode($data);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,$url);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
curl_setopt($ch, CURLOPT_USERPWD, "$username:$api_key");
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json',
    'Content-Length: ' . strlen($data_string))
);
$result=curl_exec ($ch);
curl_close ($ch);
echo $result;
?>