This a example of a curl request with basic authentication and x-www-form-urlencoded.

curl_basic_auth.php

<?php

$curl = curl_init();

$header = [];
$header[] = 'Content-type: application/x-www-form-urlencoded';
// user:password in base64
$header[] = 'Authorization: Basic xxxxxx';

curl_setopt($curl, CURLOPT_URL, 'http://url/user/add');
curl_setopt($curl, CURLOPT_HTTPHEADER, $header);
curl_setopt($curl, CURLOPT_POST,true);
curl_setopt($curl, CURLOPT_POSTFIELDS, 'id=888&email=xpto%40xpto.io&firstName=jonh&lastName=doe');

$response = curl_exec($curl);

curl_close($curl);

var_dump($response);