Example usage:

Auth0ManagementApiClient.php

<?php

namespace api_clients;

use Auth0JwtBootstrap;
use AppJson;

class Auth0ManagementApiClient
{

    /**
     * The reference to the auth0 tenant that this api client instance should target.
     * The references are the ones found in the AUTH0_APPS config variable.
     * @var
     */
    protected $auth0TenantReference;

    /**
     * @var mixed
     */
    protected $auth0AppConfig;

    /**
     * @var
     */
    protected $accessToken;

    /**
     * Auth0ManagementApiClient constructor.
     * @param $auth0TenantReference
     */
    public function __construct($auth0TenantReference)
    {
        $this->auth0TenantReference = $auth0TenantReference;
        $this->auth0AppConfig = Auth0JwtBootstrap::getConfigForSpecificAuth0TenantReference(
            $this->auth0TenantReference
        );
    }

    public function ensureAccessToken()
    {
        if (empty($this->accessToken)) {
            $this->accessToken = $this->requestAccessToken();
        }
        return $this->accessToken;
    }

    protected function curlRequest($curl)
    {

        $response = curl_exec($curl);
        $err = curl_error($curl);

        curl_close($curl);

        if ($err) {
            throw new Auth0ManagementApiRequestException("cURL Error #:" . $err);
        }

        $parsedResponse = AppJson::decode($response);

        if (!empty($parsedResponse->error)) {
            throw new Auth0ManagementApiRequestException("Error Response:" . $response);
        }

        return $parsedResponse;

    }

    public function requestAccessToken()
    {

        $endpoint = "oauth/token";

        $curl = curl_init();

        curl_setopt_array(
            $curl,
            array(
                CURLOPT_URL => "https://{$this->auth0AppConfig["domain"]}/$endpoint",
                CURLOPT_RETURNTRANSFER => true,
                CURLOPT_ENCODING => "",
                CURLOPT_MAXREDIRS => 10,
                CURLOPT_TIMEOUT => 30,
                CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
                CURLOPT_CUSTOMREQUEST => "POST",
                CURLOPT_POSTFIELDS => "{\"grant_type\":\"client_credentials\",\"client_id\": \"{$this->auth0AppConfig["management_api_client_id"]}\",\"client_secret\": \"{$this->auth0AppConfig["management_api_client_secret"]}\",\"audience\": \"https://{$this->auth0AppConfig["domain"]}/api/v2/\"}",
                CURLOPT_HTTPHEADER => array(
                    "content-type: application/json"
                ),
            )
        );

        return $this->curlRequest($curl);

    }

    public function listAllClients()
    {

        $endpoint = "api/v2/clients";

        $curl = curl_init();

        curl_setopt_array(
            $curl,
            array(
                CURLOPT_URL => "https://{$this->auth0AppConfig["domain"]}/$endpoint",
                CURLOPT_RETURNTRANSFER => true,
                CURLOPT_ENCODING => "",
                CURLOPT_MAXREDIRS => 10,
                CURLOPT_TIMEOUT => 30,
                CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
                CURLOPT_CUSTOMREQUEST => "GET",
                CURLOPT_HTTPHEADER => array(
                    "authorization: Bearer {$this->accessToken->access_token}",
                    "content-type: application/json"
                ),
            )
        );

        return $this->curlRequest($curl);

    }

    public function getUserAttributes($auth0UserId)
    {

        $endpoint = "api/v2/users/$auth0UserId";

        $curl = curl_init();

        curl_setopt_array(
            $curl,
            array(
                CURLOPT_URL => "https://{$this->auth0AppConfig["domain"]}/$endpoint",
                CURLOPT_RETURNTRANSFER => true,
                CURLOPT_ENCODING => "",
                CURLOPT_MAXREDIRS => 10,
                CURLOPT_TIMEOUT => 30,
                CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
                CURLOPT_CUSTOMREQUEST => "GET",
                CURLOPT_HTTPHEADER => array(
                    "authorization: Bearer {$this->accessToken->access_token}",
                    "content-type: application/json"
                ),
            )
        );

        return $this->curlRequest($curl);

    }

}

class Auth0ManagementApiRequestException extends \Exception
{

}