Adding subscribers to a list using Mailchimp's API v3

Based on the List Members Instance docs, the easiest way is to use a PUT request which according to the docs either "adds a new list member or updates the member if the email already exists on the list".

Furthermore apikey is definitely not part of the json schema and there's no point in including it in your json request.

Also, as noted in @TooMuchPete's comment, you can use CURLOPT_USERPWD for basic http auth as illustrated in below.

I'm using the following function to add and update list members. You may need to include a slightly different set of merge_fields depending on your list parameters.

$data = [    'email'     => 'johndoe@example.com',    'status'    => 'subscribed',    'firstname' => 'john',    'lastname'  => 'doe'];syncMailchimp($data);function syncMailchimp($data) {    $apiKey = 'your api key';    $listId = 'your list id';    $memberId = md5(strtolower($data['email']));    $dataCenter = substr($apiKey,strpos($apiKey,'-')+1);    $url = 'https://' . $dataCenter . '.api.mailchimp.com/3.0/lists/' . $listId . '/members/' . $memberId;    $json = json_encode([        'email_address' => $data['email'],        'status'        => $data['status'], // "subscribed","unsubscribed","cleaned","pending"        'merge_fields'  => [            'FNAME'     => $data['firstname'],            'LNAME'     => $data['lastname']        ]    ]);    $ch = curl_init($url);    curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $apiKey);    curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);    curl_setopt($ch, CURLOPT_TIMEOUT, 10);    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);    curl_setopt($ch, CURLOPT_POSTFIELDS, $json);                                                                                                                     $result = curl_exec($ch);    $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);    curl_close($ch);    return $httpCode;}

I got it working. I was adding the authentication to the header incorrectly:

$apikey = '<api_key>';            $auth = base64_encode( 'user:'.$apikey );            $data = array(                'apikey'        => $apikey,                'email_address' => $email,                'status'        => 'subscribed',                'merge_fields'  => array(                    'FNAME' => $name                )            );            $json_data = json_encode($data);            $ch = curl_init();            curl_setopt($ch, CURLOPT_URL, 'https://us2.api.mailchimp.com/3.0/lists/<list_id>/members/');            curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json',                                                        'Authorization: Basic '.$auth));            curl_setopt($ch, CURLOPT_USERAGENT, 'PHP-MCAPI/2.0');            curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);            curl_setopt($ch, CURLOPT_TIMEOUT, 10);            curl_setopt($ch, CURLOPT_POST, true);            curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);            curl_setopt($ch, CURLOPT_POSTFIELDS, $json_data);                                                                                                                              $result = curl_exec($ch);            var_dump($result);            die('Mailchimp executed');

These are good answers but detached from a full answer as to how you would get a form to send data and handle that response. This will demonstrate how to add a member to a list with v3.0 of the API from an HTML page via jquery .ajax().

In Mailchimp:

1. Acquire your API Key and List ID
2. Make sure you setup your list and what custom fields you want to use with it. In this case, I've set up zipcode as a custom field in the list BEFORE I did the API call.
3. Check out the API docs on adding members to lists. We are using the create method which requires the use of HTTP POST requests. There are other options in here that require PUT if you want to be able to modify/delete subs.

HTML:

<form id="pfb-signup-submission" method="post">  <div class="sign-up-group">    <input type="text" name="pfb-signup" id="pfb-signup-box-fname" class="pfb-signup-box" placeholder="First Name">    <input type="text" name="pfb-signup" id="pfb-signup-box-lname" class="pfb-signup-box" placeholder="Last Name">    <input type="email" name="pfb-signup" id="pfb-signup-box-email" class="pfb-signup-box" placeholder="youremail@example.com">    <input type="text" name="pfb-signup" id="pfb-signup-box-zip" class="pfb-signup-box" placeholder="Zip Code">  </div>  <input type="submit" class="submit-button" value="Sign-up" id="pfb-signup-button"></a>  <div id="pfb-signup-result"></div></form>

Key things:

1. Give your <form> a unique ID and don't forget the method="post" attribute so the form works.
2. Note the last line #signup-result is where you will deposit the feedback from the PHP script.

PHP:

<?php  /*   * Add a 'member' to a 'list' via mailchimp API v3.x   * @ http://developer.mailchimp.com/documentation/mailchimp/reference/lists/members/#create-post_lists_list_id_members   *   * ================   * BACKGROUND   * Typical use case is that this code would get run by an .ajax() jQuery call or possibly a form action   * The live data you need will get transferred via the global $_POST variable   * That data must be put into an array with keys that match the mailchimp endpoints, check the above link for those   * You also need to include your API key and list ID for this to work.   * You'll just have to go get those and type them in here, see README.md   * ================   */  // Set API Key and list ID to add a subscriber  $api_key = 'your-api-key-here';  $list_id = 'your-list-id-here';  /* ================   * DESTINATION URL   * Note: your API URL has a location subdomain at the front of the URL string   * It can vary depending on where you are in the world   * To determine yours, check the last 3 digits of your API key   * ================   */  $url = 'https://us5.api.mailchimp.com/3.0/lists/' . $list_id . '/members/';  /* ================   * DATA SETUP   * Encode data into a format that the add subscriber mailchimp end point is looking for   * Must include 'email_address' and 'status'   * Statuses: pending = they get an email; subscribed = they don't get an email   * Custom fields go into the 'merge_fields' as another array   * More here: http://developer.mailchimp.com/documentation/mailchimp/reference/lists/members/#create-post_lists_list_id_members   * ================   */  $pfb_data = array(    'email_address' => $_POST['emailname'],    'status'        => 'pending',    'merge_fields'  => array(      'FNAME'       => $_POST['firstname'],      'LNAME'       => $_POST['lastname'],      'ZIPCODE'     => $_POST['zipcode']    ),  );  // Encode the data  $encoded_pfb_data = json_encode($pfb_data);  // Setup cURL sequence  $ch = curl_init();  /* ================   * cURL OPTIONS   * The tricky one here is the _USERPWD - this is how you transfer the API key over   * _RETURNTRANSFER allows us to get the response into a variable which is nice   * This example just POSTs, we don't edit/modify - just a simple add to a list   * _POSTFIELDS does the heavy lifting   * _SSL_VERIFYPEER should probably be set but I didn't do it here   * ================   */  curl_setopt($ch, CURLOPT_URL, $url);  curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $api_key);  curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);  curl_setopt($ch, CURLOPT_TIMEOUT, 10);  curl_setopt($ch, CURLOPT_POST, 1);  curl_setopt($ch, CURLOPT_POSTFIELDS, $encoded_pfb_data);  curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);  $results = curl_exec($ch); // store response  $response = curl_getinfo($ch, CURLINFO_HTTP_CODE); // get HTTP CODE  $errors = curl_error($ch); // store errors  curl_close($ch);  // Returns info back to jQuery .ajax or just outputs onto the page  $results = array(    'results' => $result_info,    'response' => $response,    'errors' => $errors  );  // Sends data back to the page OR the ajax() in your JS  echo json_encode($results);?>

Key things:

1. CURLOPT_USERPWD handles the API key and Mailchimp doesn't really show you how to do this.
2. CURLOPT_RETURNTRANSFER gives us the response in such a way that we can send it back into the HTML page with the .ajax() success handler.
3. Use json_encodeon the data you received.

JS:

// Signup form submission$('#pfb-signup-submission').submit(function(event) {  event.preventDefault();  // Get data from form and store it  var pfbSignupFNAME = $('#pfb-signup-box-fname').val();  var pfbSignupLNAME = $('#pfb-signup-box-lname').val();  var pfbSignupEMAIL = $('#pfb-signup-box-email').val();  var pfbSignupZIP = $('#pfb-signup-box-zip').val();  // Create JSON variable of retreived data  var pfbSignupData = {    'firstname': pfbSignupFNAME,    'lastname': pfbSignupLNAME,    'email': pfbSignupEMAIL,    'zipcode': pfbSignupZIP  };  // Send data to PHP script via .ajax() of jQuery  $.ajax({    type: 'POST',    dataType: 'json',    url: 'mailchimp-signup.php',    data: pfbSignupData,    success: function (results) {      $('#pfb-signup-box-fname').hide();      $('#pfb-signup-box-lname').hide();      $('#pfb-signup-box-email').hide();      $('#pfb-signup-box-zip').hide();      $('#pfb-signup-result').text('Thanks for adding yourself to the email list. We will be in touch.');      console.log(results);    },    error: function (results) {      $('#pfb-signup-result').html('<p>Sorry but we were unable to add you into the email list.</p>');      console.log(results);    }  });});

Key things:

1. JSON data is VERY touchy on transfer. Here, I am putting it into an array and it looks easy. If you are having problems, it is likely because of how your JSON data is structured. Check this out!
2. The keys for your JSON data will become what you reference in the PHP _POST global variable. In this case it will be _POST['email'], _POST['firstname'], etc. But you could name them whatever you want - just remember what you name the keys of the data part of your JSON transfer is how you access them in PHP.
3. This obviously requires jQuery ;)