$user = 'xxxxxxxxxxxxxxxxxxxxxxxx';
$pass = '';
$data = array(
        "email" => "Baralho@gmail.com",
        "due_date" => "2018-10-08",
        "items" => array(
                "description"=>"carro",
                "quantity"=> 1,
                "price_cents"=>150000
                ),
        "payer"=>array(
                "cpf_cnpj"=>"644.620.920-74",
                "name"=>"Jeison",
                "phone_prefix"=>"31",
                "phone"=>"991872520",
                "address"=>array(
                        "zip_code"=>"30520240",
                        "number"=>"109",
                        "street"=>"blenda"
                        )
                 )
        );
$data_string=json_encode($data);
$ch = curl_init();
curl_setopt( $ch, CURLOPT_URL, 'https://api.iugu.com/v1/invoices/' );
curl_setopt( $ch, CURLOPT_HEADER, 1 );
curl_setopt( $ch, CURLOPT_HTTPHEADER, array('Authorization: Basic ' . base64_encode( $user . ':' . $pass ) ) );
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch,CURLOPT_POST, TRUE);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
$return=curl_exec( $ch );
curl_close( $ch );
header("Content-Type: text/json; charset=utf8");`

//demais itens omitidos
$data = array(
    "items" => array(
        array(
            "description"=>"carro",
            "quantity"=> 1,
            "price_cents"=>150000
        )
    )
);

//demais itens omitidos
$data = array(
    "items" => [
        [
            "description"=>"carro",
            "quantity"=> 1,
            "price_cents"=>150000
        ]
    ]
);