<?php

//AccessToken.php

class AccessToken
{
        public $client_id = '';  
        public $client_secret = '';  
        public $grant_type = '';
}

$accessToken = new AccessToken();
$accessToken->client_id = 'YOUR API KEY HERE';
$accessToken->client_secret = 'YOUR API SECRET HERE';
$accessToken->grant_type = 'client_credentials';

$requestParameters = array('request'=>$accessToken);

$wsdl = "https://apicurrent-app.booker.ninja/WebService4/soap/CustomerService.svc?wsdl";
$endpoint = "https://apicurrent-app.booker.ninja/WebService4/soap/CustomerService.svc";

$client = new SoapClient($wsdl, array('location'=>$endpoint));
$response = $client->GetAccessToken($requestParameters);

var_dump($response);

?>