Crulより簡単で、シンプルに書ける

guzzlehttp/guzzle 6.3.3

composer require guzzlehttp/guzzle

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use GuzzleHttp\Client;

class XmlRpc extends Controller
{
    protected $url;
    protected $user;
    protected $password;
    public function __construct($url='', $user='', $password='')
    {
        $this->url      = $url;
        $this->user     = $user;
        $this->password = $password;
    }
    public function send($method, $params = [])
    {
        $auth = ['login' => $this->user, 'password' => $this->password];
        $params = [$auth, $params];
        $body     = xmlrpc_encode_request($method, $params);
        //print_r($body);
        $response = (new Client)->post($this->url, [
            'body' => $body,
            'auth' => [$this->user, $this->password]
        ]);
        return $response->getBody()->getContents();
    }
}

$XmlRcp =  new App\Http\Controllers\XmlRpc($url, $user, $pass);
$XmlRcp->send('partner10.getKeyInfo', 'PLSK.XXXXXXXX.XXXX')