Mailchimp API Integration in PHP, FNAME and LNAME not passing - php

Question

I've been trying to integrate the Mailchimp API in PHP on our website, and I cannot seem to get mailchimp to take the FNAME and the LNAME.
The email always gets passed through to mailchimp and they are added to the list but the names simply don't and are always blank.
Things I have tried:
Using static names such as Dave in place of $_POST[FirstNAME] etc. but still no luck
Using MERGE1 and MERGE2 which are the alternate names for FNAME and LNAME.
Sending the email along with the FNAME and LNAME in the array for merge_vars
Putting the array in directly or as it is below within a variable ($Merge).
require('../MailCHIMP_API_PHP/Mailchimp.php');
$Mailchimp = new Mailchimp( $api_key );
$Mailchimp_Lists = new Mailchimp_Lists( $Mailchimp );
$Merge = array('FNAME' => $_POST[FirstNAME], 'LNAME' => $_POST[LastNAME]);
$subscriber = $Mailchimp_Lists->subscribe( $list_id, array(
'email' => htmlentities($_POST[Email]),
'merge_vars' => $Merge
));
if ( ! empty( $subscriber['leid'] ) ) {
//echo "success";
} else {
//echo "fail";
}
As always there is probably something simple I am missing but I have been staring at this code for so long I obviously can't see it!

Answer 1

This issue was solved so thought I might as well post an answer. Here is the code that works.
require('../MailCHIMP_API_PHP/Mailchimp.php');
$Mailchimp = new Mailchimp( $api_key );
$subscriber = $Mailchimp->call('lists/subscribe',
array(
'id' => $list_id,
'email' => array('email' => htmlentities($_POST[Email])),
'merge_vars' => array('FNAME' => htmlentities($_POST[FirstNAME]), 'LNAME' => htmlentities($_POST[LastNAME])),
'double_optin' => false
));
if ( ! empty( $subscriber['leid'] ) ) {
//echo "success";
} else {
//echo "fail";
}