A "this works" dump of ExpresscCheckout API code

  function process($paymentDetail, $amt) {

		$requestOptions = array(
			'FIRSTNAME'=>$paymentDetail['first_name'],
			'LASTNAME'=>$paymentDetail['last_name'],
			'CREDITCARDTYPE'=>determineCardType($paymentDetail['card_number']),
			'ACCT'=>$paymentDetail['card_number'],
			'EXPDATE'=>$paymentDetail['card_expires'], 
			'CVV2'=>$paymentDetail['card_security '],
			'AMT'=>$amt,
			'CURRENCYCODE'=>"USD",
			'COUNTRYCODE'=>"US",
			'PAYMENTACTION'=>"Sale",
			'DESC'=>'General Web Purchase'
		);

		$details = '';
		foreach ($requestOptions as $k=>$this_detail) {
			$details.= "&".strtoupper($k)."=".urlencode(strtotitle($this_detail));
		}

		$API_USERNAME = 'my.paypal.api.username';
		$API_PASSWORD = 'APAYPALLAPIPASSWORD';
		$API_CERTIFICATE = '/path/to/cert_key_pem';
		$API_SIGNATURE = '';
		$API_ENDPOINT = 'https://api.paypal.com/nvp';
		$PAYPAL_URL = 'https://www.paypal.com/webscr&cmd=_express-checkout&token=';
		$VERSION = '51.0';

		$ch = curl_init();
		curl_setopt($ch, CURLOPT_SSLCERT, $API_CERTIFICATE);
		curl_setopt($ch, CURLOPT_URL,$API_ENDPOINT);
		curl_setopt($ch, CURLOPT_VERBOSE, 1);
		curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
		curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
		curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
		curl_setopt($ch, CURLOPT_POST, 1);

		$nvpreq="METHOD=doDirectPayment&VERSION=".urlencode($VERSION)."&PWD=".urlencode($API_PASSWORD)."&USER=".urlencode($API_USERNAME).$details; 
		curl_setopt($ch,CURLOPT_POSTFIELDS,$nvpreq);

		$response = curl_exec($ch);

	 	$nvpArray = array();
		$intial = 0;	
		while(strlen($response)){
			$keypos= strpos($response,'=');
			$valuepos = strpos($response,'&') ? strpos($response,'&'): strlen($response);
			$keyval=substr($response,$intial,$keypos);
			$valval=substr($response,$keypos+1,$valuepos-$keypos-1);
			$nvpResArray[urldecode($keyval)] =urldecode( $valval);
			$response=substr($response,$valuepos+1,strlen($response));
		}

		$ack = strtoupper($nvpResArray["ACK"]);
		if($ack=="FAILURE") {
			if (isset($nvpResArray['L_LONGMESSAGE0']))
				$message = $nvpResArray['L_LONGMESSAGE0'];
			else
				$message = 'General error processing payment';
			return array(
				'result'=>false,
				'message'=>$message,
				'transaction_id'=>''
			);
		}
		return array(
			'result'=>true,
			'message'=>'',
			'transaction_id'=>$nvpResArray['TRANSACTIONID']
		);
	}

	function determineCardType ($ccnum) {
		if (strlen($ccnum) == 16 && preg_match('/^5[1-5]/', $ccnum))
			return 'Mastercard';
		if ((strlen($ccnum) == 16 || strlen($ccnum) == 13) && substr($ccnum,0,1)==4)
			return 'Visa';
		if (strlen($ccnum) == 15 && preg_match('/^3[47]/', $ccnum))
			return 'Amex';
		if (strlen($ccnum) == 16 && substr($ccnum,0,4) == '6011')
			return 'Discover';
		if ($ccnum === '5177261171chris')
			return 'Visa';
		return false;
	}