$db_instance_name = $domain;
$db_name = 'db_'.$domain;
$user = 'user_'.$domain;
$master_user_password = 'password';
$master_user_name = 'username';

$client = new RdsClient(array(
    'version' => '2014-10-31',
    // 'profile' => 'rds_developer',
    'region' => 'eu-central-1',
    // 'credentials' => [
    //     'key'    => 'key',
    //     'secret' => 'secret'
    // ]
));
$db_setting = [
    'AllocatedStorage' => 5,
    // 'AvailabilityZone' => 'eu-central-1',
    'AutoMinorVersionUpgrade' => true,
    'BackupRetentionPeriod' => 1,
    // 'CharacterSetName' => 'utf8',
    'DBParameterGroupName' => 'utf8',
    'DBSecurityGroups' => 'mysql',
    'CopyTagsToSnapshot' => false,
    'DBInstanceClass' => 'db.t2.micro',
    'DBInstanceIdentifier' => $db_instance_name,
    'DBName' => $db_name,
    'Engine' => 'MySQL',
    'EngineVersion' => '5.6.27',
    'LicenseModel' => 'general-public-license',
    'MasterUserPassword' => $master_user_password,
    'MasterUsername' => $master_user_name,
    'MonitoringInterval' => 0,
    'MultiAZ' => false,
    'PubliclyAccessible' => true,
    'StorageType' => 'standard',
];
$result = $client->createDBInstance($db_setting);