##ユーザー登録

<?php

$data = [
   "jsonrpc"=> "2.0",
   "method"=> "User.create",
   "params"=> [
     "username"=> "yamada",
     "password"=> "password"
   ],
   "id" => 1
];

$header = [
    'X-Monaca-Backend-Management-API-Key: ************** ' // APIキー
];

$curl = curl_init();

curl_setopt($curl, CURLOPT_URL, 'https://cloud.monaca.mobi/manage/json-rpc/************'); //　エンドポイント
curl_setopt($curl, CURLOPT_HTTPHEADER, $header);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

$response = curl_exec($curl);
curl_close($curl);

$result = json_decode($response, true); 

var_export($result);

##ユーザー取得
ユーザー名からidを取得する

<?php

$username = 'yamada';

$data = [
   "jsonrpc"=> "2.0",
   "method"=> "User.list",
   "params"=> [
     "page"=> 1,
     "itemsInPage"=> 100,
     "sortProperty"=> "_username",
     "sortOrder" => "asc",
     "nameFilter" => $username
   ],
   "id" => 1
];

$header = [
    'X-Monaca-Backend-Management-API-Key: ********** '
];

$curl = curl_init();

curl_setopt($curl, CURLOPT_URL, 'https://cloud.monaca.mobi/manage/json-rpc/***********');
curl_setopt($curl, CURLOPT_HTTPHEADER, $header);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

$response = curl_exec($curl);
curl_close($curl);

$response = json_decode($response);
$result = $response->result;

$id = "";
for ($i = 0; $i < $result->totalItems; $i++) {
    if ($result->items[$i]->_username == $username) {
        $id = $result->items[$i]->_id;
        break;
    }
}
echo $id;

ネットを探してもサンプルが少なかったので。