$xmlapi = new xmlapi($host);
$xmlapi->password_auth("".$root_user."","".$root_pass."");
$xmlapi->set_debug(1);
$xmlapi->set_output('array');
$xmlapi->set_port('2083');
//create database
$createdb = $xmlapi->api1_query($root_user, "Mysql", "adddb", array($dbname));

URL: https://delta1.powerstorm.net:2083/xml-api/cpanel
DATA: cpanel_xmlapi_user=root&cpanel_xmlapi_module=Mysql&cpanel_xmlapi_func=adddb&cpanel_xmlapi_apiversion=1&arg-0=site
Authentication Header: Authorization: Basic cm9vdDphY3Jtp3MxOTY1

RESPONSE:
 <?xml version="1.0" ?>
<cpanelresult>
    <error>Access denied</error>
    <data>
        <result>0</result>
        <reason>Access denied</reason>
    </data>
</cpanelresult>

require("xmlapi.php");

$opts = [
    "userName"   => "UserUserName",      //+++ Replace UserUserName
    "password"   => "UserPassword",      //+++ Replace UserPassword
    "dbPassword" => "DatabasePassword",  //+++ Replace DatabasePassword
];

$xmlapi = new xmlapi("domainName");
$xmlapi->set_port( 2083 );
$xmlapi->password_auth($opts['userName'],$opts['password']);

$cpaneluser=$opts['userName'];
$databasename="dbName";
$databaseuser="dbUserName";
$databasepass=$opts['dbPassword'];

// database creation
$createdb = $xmlapi->api1_query($cpaneluser, "Mysql", "adddb", array($databasename));
// user creation
$usr = $xmlapi->api1_query($cpaneluser, "Mysql", "adduser", array($databaseuser, $databasepass));
// adds user to database
$addusr = $xmlapi->api1_query($cpaneluser, "Mysql", "adduserdb", array("".$cpaneluser."_".$databasename."", "".$cpaneluser."_".$databaseuser."", 'all'));

// user creation
$usr = $xmlapi->api1_query($cpaneluser, "Mysql", "adduser", array($databaseuser, $databasepass));

// database creation
$createdb = $xmlapi->api1_query($cpaneluser, "Mysql", "adddb", array($databasename));