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.
$data = [
'email' => '[email protected]',
'status' => 'subscribed',
'firstname' => 'john',
'lastname' => 'doe'
];
syncMailchimp($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;
}