Twitter API returns error code 32 - php

Question

We're currently working with the Twitter API and failed to sucessfully send a tweet to their API. It's always exiting with code 32 in both Java and PHP (made a PHP prototype just for sending the request). I guess I'm missing something about the OAuth1 specification here.
The PHP prototype we're using:
<?PHP
require_once("cURL.php");
$headers = [];
$postParams = [];
$signatureParams = [];
$authString = "";
$URL = "https://api.twitter.com/1.1/statuses/update.json";
$method = "POST";
$token = "someValidToken";
$tokenSecret = "someValidTokenSecret";
$timestamp = time();
$status = "Hi, Twitter!";
$signatureParams['status'] = $status;
$clientID = "someClientID";
$clientSecret = "someClientSecret";
$signatureParams['oauth_consumer_key'] = urlencode($clientID);
$signatureParams['oauth_nonce'] = urlencode(base64_encode($timestamp));
$signatureParams['oauth_signature_method'] = urlencode("HMAC-SHA1");
$signatureParams['oauth_timestamp'] = $timestamp;
$signatureParams['oauth_token'] = urlencode($token);
$signatureParams['oauth_version'] = urlencode("1.0");
$oauthSignature = generateSignature();
$signatureParams["oauth_signature"] = $oauthSignature;
unset($signatureParams['status']);
ksort($signatureParams);
$count = 0;
$authStringLoop = "";
foreach($signatureParams as $key => $signatureParam)
{
if($count++ == 0)
{
$authStringLoop .= "OAuth ";
}
$authStringLoop .= urlencode($key);
$authStringLoop .= "=\"";
$authStringLoop .= urlencode($signatureParam)."\",";
}
$authStringLoop = substr($authStringLoop,0,strlen($authStringLoop)-1);
echo "</br></br>";
echo "AuthString loop: $authStringLoop</br></br>";
$headers = array(
"Authorization: $authStringLoop"
);
var_dump($headers).'</br>';
var_dump(POST($URL,json_encode($status),$headers,0));
function generateSignature()
{
global $clientSecret;
global $signatureParams;
global $URL;
global $method;
global $clientSecret;
global $clientID;
global $tokenSecret;
ksort($signatureParams);
$parameterString = "";
foreach($signatureParams as $key => $signatureParam)
{
$parameterString .= urlencode($key."=".$signatureParam."&");
}
$signingBase = strtoupper($method)."&".urlencode($URL)."&".$parameterString;
$signingKey = urlencode($clientSecret)."&".urlencode($tokenSecret);
var_dump($signingBase);
echo '</br>';
echo "Base64 encoded: ".base64_encode(hash_hmac("SHA1",$signingBase,$signingKey,TRUE));
echo '</br>';
echo "URL encoded: ".urlencode(base64_encode(hash_hmac("SHA1",$signingBase,$signingKey,TRUE)))."</br>";
return base64_encode(hash_hmac("SHA1",$signingBase,$signingKey,TRUE));
}
?>
My guess that there is something wrong with the way the signature is generated. However, all I found was to sort it alphabetically which should be done with the ksort() call. What do we miss here?