AddOrder

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://www.example.com/includes/api.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,
    http_build_query(
        array(
            'action' => 'AddOrder',
            // See https://developers.whmcs.com/api/authentication
            'username' => 'IDENTIFIER_OR_ADMIN_USERNAME',
            'password' => 'SECRET_OR_HASHED_PASSWORD',
            'clientid' => '1',
            'pid' => array(1,1),
            'qty' => array(2, 3),
            'domain' => array('domain1.com', 'dómáin2.com'),
            'idnlanguage' => array('', 'fre'),
            'billingcycle' => array('monthly','semiannually'),
            'addons' => array('1,3,9', ''),
            '$addonsqty' => array('2,4,9', ''),
            'customfields' => array(base64_encode(serialize(array("1" => "Google"))), base64_encode(serialize(array("1" => "Google")))),
            'configoptions' => array(base64_encode(serialize(array("1" => 999))), base64_encode(serialize(array("1" => 999)))),
            'domaintype' => array('register', 'register'),
            'regperiod' => array(1, 2),
            'dnsmanagement' => array(0 => false, 1 => true),
            'nameserver1' => 'ns1.demo.com',
            'nameserver2' => 'ns2.demo.com',
            'paymentmethod' => 'mailin',
            'servicerenewals' => array(3, 10),
            'addonrenewals' => array(3, 10),
            'responsetype' => 'json',
        )
    )
);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
curl_close($ch);

$command = 'AddOrder';
$postData = array(
    'clientid' => '1',
    'pid' => array(1,1),
    'qty' => array(2, 3),
    'domain' => array('domain1.com', 'dómáin2.com'),
    'idnlanguage' => array('', 'fre'),
    'billingcycle' => array('monthly','semiannually'),
    'addons' => array('1,3,9', ''),
    '$addonsqty' => array('2,4,9', ''),
    'customfields' => array(base64_encode(serialize(array("1" => "Google"))), base64_encode(serialize(array("1" => "Google")))),
    'configoptions' => array(base64_encode(serialize(array("1" => 999))), base64_encode(serialize(array("1" => 999)))),
    'domaintype' => array('register', 'register'),
    'regperiod' => array(1, 2),
    'dnsmanagement' => array(0 => false, 1 => true),
    'nameserver1' => 'ns1.demo.com',
    'nameserver2' => 'ns2.demo.com',
    'paymentmethod' => 'mailin',
    'servicerenewals' => array(3, 10),
    'addonrenewals' => array(3, 10),
);
$adminUsername = 'ADMIN_USERNAME'; // Optional for WHMCS 7.2 and later

$results = localAPI($command, $postData, $adminUsername);
print_r($results);

{
    "result": "success",
    "orderid": "1",
    "serviceids": "1,2",
    "addonids": "1,2,3",
    "domainids": "1,2",
    "invoiceid": "1"
}