I registered on here just to respond to this and comment about what total crap the PayPal docs, sandbox, and code examples are.  Then I was greeted with this, which sums it up just about perfectly:

"Your password must be 6 or more characters, contain at least one lowercase character, one uppercase character, one digit, and no repeated sequence of characters of any length. Your password can't be a subset of your login name."

Seriously Paypal!?  Who the F is running your show?  The day google checkout localizes currencies in other countries will be one of the happiest days of my life.

So yes, after slogging through their docs and getting my mobile NVP token response working, I was also greeted with mysterious 500 server errors.  Found this thread after a while.  Thanks guys.

Oh, and by the way?  This little gem of PHP code?

/** This function will take NVPString and convert it to an Associative Array and it will decode the response.
  * It is usefull to search for a particular key and displaying arrays.
  * @nvpstr is NVPString.
  * @nvpArray is Associative Array.
  */

function deformatNVP($nvpstr)
{

    $intial=0;
     $nvpArray = array();


    while(strlen($nvpstr)){
        //postion of Key
        $keypos= strpos($nvpstr,'=');
        //position of value
        $valuepos = strpos($nvpstr,'&') ? strpos($nvpstr,'&'): strlen($nvpstr);

        /*getting the Key and Value values and storing in a Associative Array*/
        $keyval=substr($nvpstr,$intial,$keypos);
        $valval=substr($nvpstr,$keypos+1,$valuepos-$keypos-1);
        //decoding the respose
        $nvpArray[urldecode($keyval)] =urldecode( $valval);
        $nvpstr=substr($nvpstr,$valuepos+1,strlen($nvpstr));
     }
    return $nvpArray;
}

Are you SERIOUS?

function welcomeToPHP4.0($morons)

{

parse_str($morons, $returnArray);

return $returnArray;

}

You're welcome.

FFS.