use correct seperator character

maybe

at least according to https://github.com/sypper-pit/php-matrix-register/blob/main/register.php#L40

index.php

@@ -46,7 +46,7 @@ if (isset($_GET['secret']) && $_GET['secret'] === REGISTRATION_PASSWORD) {
         'password' => $_POST['password'],
         'admin' => 'notadmin'
     ];
-    $content['mac'] = hash_hmac('sha1', $content['nonce']."\s".$content['username']."\s".$content['password']."\s".$content['admin'], REGISTRATION_SHARED_SECRET);
+    $content['mac'] = hash_hmac('sha1', $content['nonce']."\0".$content['username']."\0".$content['password']."\0".$content['admin'], REGISTRATION_SHARED_SECRET);
 
     $ch = curl_init();
     curl_setopt($ch, CURLOPT_URL, SYNAPSE_URL.'/_synapse/admin/v1/register');