Resolvido!
<?php
$url = 'https://servidor.com/xx';
$ch = curl_init($url);
curl_setopt($ch, CURLOPT_USERPWD, 'usuario:senha');
curl_setopt($ch, CURLOPT_TIMEOUT, 5);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$data = json_decode(curl_exec($ch),true); // <== Aqui
curl_close($ch);
$id = $data['id']; // <== Aqui
if ($id == $variavel) {
echo 0;
} else {
echo 1;
}
?>