Authentication failed. API credentials are incorrect - paypal

Question

I've been trying to create a Paypal adaptive payments API call to the sandbox all day now without success. I've read dozends of tutorials, SE threads etc but nothing I tried worked. I keep getting the "Authentication failed. API credentials are incorrect." error message.
The following is the code I'm using:
$baseurl = 'https://svcs.sandbox.paypal.com/AdaptivePayments/Pay';
define('PAYPAL_API_USER', '<snip>');
define('PAYPAL_API_PWD', '<snip>');
define('PAYPAL_API_SIGNATURE', '<snip>');
define('PAYPAL_API_APPLICATION_ID', 'APP-80W284485P519543T');
$returnurl = "<snip>";
$cancelurl = "<snip>";
$data = array(
"actionType" => "PAY",
"currencyCode" => "USD",
"receiverList" => array(
"receiver" => array(
array(
"amount" => "1.00",
"email" => "<snip>"
),
)
),
"returnUrl" => $returnurl,
"cancelUrl" => $cancelurl,
"requestEnvelope" => array(
"errorLangauge" => "en_US",
"detailLevel" => "ReturnAll",
)
);
$headers = array(
"X-PAYPAL-SECURITY-USERID: ".PAYPAL_API_USER,
"X-PAYPAL-SECURITY-PASSWORD: ".PAYPAL_API_PWD,
"X-PAYPAL-SECURITY-SIGNATURE: ".PAYPAL_API_SIGNATURE,
"X-PAYPAL-SECURITY-REQUEST-DATA-FORMAT: JSON",
"X-PAYPAL-SECURITY-RESPONSE-DATA-FORMAT: JSON",
"X-PAYPAL-SECURITY-APPLICATION-ID: ".PAYPAL_API_APPLICATION_ID,
);
$ch = curl_init();
curl_setopt ($ch, CURLOPT_URL, $baseurl);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt ($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt ($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt ($ch, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt ($ch, CURLOPT_HEADER, $headers);
print_r(curl_exec($ch));
I created several test accounts under "Sandbox Test Accounts" on developer.paypal.com. I tried using their username + password + signature (from the API Credentials tab) in the code above.
Always getting "Authentication failed. API credentials are incorrect."
Edit 1:
On this page: https://developer.paypal.com/docs/classic/adaptive-payments/gs_AdaptivePayments/ they have a similar curl command. I run this command:
curl -s --insecure -H "X-PAYPAL-SECURITY-USERID: caller_1312486258_biz_api1.gmail.com" -H "X-PAYPAL-SECURITY-PASSWORD: 1312486294" -H "X-PAYPAL-SECURITY-SIGNATURE: AbtI7HV1xB428VygBUcIhARzxch4AL65.T18CTeylixNNxDZUu0iO87e" -H "X-PAYPAL-REQUEST-DATA-FORMAT: JSON" -H "X-PAYPAL-RESPONSE-DATA-FORMAT: JSON" -H "X-PAYPAL-APPLICATION-ID: APP-80W284485P519543T" https://svcs.sandbox.paypal.com/AdaptivePayments/Pay -d "{\"actionType\":\"PAY\", \"currencyCode\":\"USD\", \"receiverList\":{\"receiver\":[{\"amount\":\"1.00\",\"email\":\"rec1_1312486368_biz#gmail.com\"}]}, \"returnUrl\":\"http://www.example.com/success.html\", \"cancelUrl\":\"http://www.example.com/failure.html\", \"requestEnvelope\":{\"errorLanguage\":\"en_US\", \"detailLevel\":\"ReturnAll\"}}"
in a terminal window and got a payment created success response.
So next I switched the userid/password/signature in this command with the ones from the original question, and guess what, that worked too!
Finally I used the userid/password/signature from the example in the PHP curl code from my original question and again got the "Authentication failed. API credentials are incorrect." message.
To sum things up both userid/password/signature combinations (mine and Paypal's) generate a success response if run via terminal, but both produce authentication failure errors when run via my PHP CURL code. So there is definitely something wrong in my original code, question is WHAT??
Edit 2:
Still can't get it to work, but I'm thinking it's something to do with the headers bit. Here is why: take a look at the full response I'm getting:
HTTP/1.1 200 OK
Date: Tue, 13 Oct 2015 11:51:23 GMT
Server: Apache
X-EBAY-SOA-REQUEST-ID: 150610a6-a690-a488-6781-12e6ffa2f8dd!AdaptivePayments!10.72.134.120![]
X-PAYPAL-SERVICE-VERSION: 1.0.0
X-PAYPAL-SERVICE-NAME: {http://svcs.paypal.com/types/ap}AdaptivePayments
X-EBAY-SOA-RESPONSE-DATA-FORMAT: XML
X-PAYPAL-OPERATION-NAME: Pay
CACHE-CONTROL: no-cache
X-PAYPAL-ERROR-RESPONSE: TRUE
X-EBAY-SOA-MESSAGE-PROTOCOL: NONE
Set-Cookie: Apache=10.72.108.11.1444737084008373; path=/; expires=Thu, 05-Oct-45 11:51:24 GMT
Vary: Accept-Encoding
Paypal-Debug-Id: 32922591f3d33
Set-Cookie: X-PP-SILOVER=name%3DSANDBOX3.APIT.1%26silo_version%3D880%26app%3Dadaptivepaymentspartaweb_api3t%26TIME%3D1005591638; domain=.paypal.com; path=/; Secure; HttpOnly
Set-Cookie: X-PP-SILOVER=; Expires=Thu, 01 Jan 1970 00:00:01 GMT
Connection: close
Transfer-Encoding: chunked
Content-Type: text/xml;charset=UTF-8
<?xml version='1.0' encoding='UTF-8'?><ns3:FaultMessage xmlns:ns3="http://svcs.paypal.com/types/common" xmlns:ns2="http://svcs.paypal.com/types/ap"><responseEnvelope><timestamp>2015-10-13T04:51:24.021-07:00</timestamp><ack>Failure</ack><correlationId>e133487976d4b</correlationId><build>17820627</build></responseEnvelope><error><errorId>520003</errorId><domain>PLATFORM</domain><subdomain>Application</subdomain><severity>Error</severity><category>Application</category><message>Authentication failed. API credentials are incorrect.</message></error></ns3:FaultMessage>
It's XML why I specified JSON as the response data format. So if the response format line is ignored or not properly read I guess the same thing is messing the login details. Hope this is a good clue...

Answer 1

If you are 100% certain that the API Credentials for a business Sandbox account are correct then verify you have the correct App id. The APP Id needs to be the APP id from the application you submitted for Adaptive Payments.
If you have not submitted an application yet here are the instructions:
How to Submit a Developer Application
If you have already applied for Adaptive Payments go to your list of applications, and click on your application to verify the correct App id.