I am using this for the PayPal Express Checkout API, but I get an error:

```
ACK=Failure&VERSION=78&BUILD=000000&L_ERRORCODE0=10002&L_SHORTMESSAGE0=Security%20error&L_LONGMESSAGE0=Security%20header%20is%20not%20valid&L_SEVERITYCODE0=Error

```

Code:

```
$request = 'USER=myuser.de&PWD=mypass&SIGNATURE=mysig&METHOD=SetExpressCheckout&VERSION=78&PAYMENTREQUEST_0_PAYMENTACTION=SALE&PAYMENTREQUEST_0_AMT=19&PAYMENTREQUEST_0_CURRENCYCODE=EUR&cancelUrl=http://xyz.biz/index.php?id=16&returnUrl=http://xyz.biz/abc/';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://api-3t.sandbox.paypal.com/nvp");
curl_setopt($ch, CURLOPT_POSTFIELDS, $request);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_TIMEOUT, 20);
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
$result = curl_exec ($ch);
curl_close ($ch);

```

What can I do?