Accessing the PayPal ButtonManager API with PHP cURL - php

Question

I'm trying to access the PayPal ButtonManager API. I'm getting error 10002, Authentication failed. I've double/triple checked all the credentials, and regenerated them too. I don't think the AppID should be necessary; it doesn't work with or without it anyway.
Exact error message text:
L_ERRORCODE0=10002&L_SHORTMESSAGE0=Authentication/Authorization Failed&L_LONGMESSAGE0=You do not have permissions to make this API call&L_SEVERITYCODE0=Error
//ButtonManager API
$ret = ch_post();
error_log($ret);
echo urldecode($ret);
function ch_post(){
//API Credentials
$accID = urlencode("accID");
$username = urlencode("username_api1.website");
$password = urlencode("password");
$signature = urlencode("signature");
$appID = urlencode("APP-ID");
$endpoint = "https://api-3t.sandbox.paypal.com/nvp";
$certpath = "C:\certpath.pem";
$ch_headers = array(
"USER"=>$username,
"PWD"=>$password,
"SIGNATURE"=>$signature,
"APPID"=>$appID,
"VERSION"=>"51.0"
);
$ch_params = array(
"METHOD"=>urlencode("BMCreateButton"),
"OTHERPARAMS"=>urlencode("OTHER")
);
$ch = curl_init($endpoint);
curl_setopt($ch,CURLOPT_HTTPHEADER,http_build_query($ch_headers));
curl_setopt($ch,CURLOPT_CAINFO,$certpath);
curl_setopt($ch,CURLOPT_POSTFIELDS,http_build_query($ch_params));
curl_setopt($ch,CURLOPT_POST,TRUE);
curl_setopt($ch,CURLOPT_HEADER,TRUE);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,TRUE);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch,CURLOPT_SSLVERSION, 6);
$cexec = curl_exec($ch);
if(!$cexec) {
$response = "Failed: ".curl_error($ch)."(".curl_errno($ch).")";
curl_close($ch);
return $response;
}
curl_close($ch);
return $cexec;
}