下面是代码，方便复制

get_code函数里面
 $def_url  = '<br /><form style="text-align:center;" action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post" target="_blank">' .   // 不能省略
respond 函数里面
// post back to PayPal system to validate
$header = "POST /cgi-bin/webscr HTTP/1.0\r\n";
$header .= "HOST: www.sandbox.paypal.com\r\n";
$header .= "Content-Type: application/x-www-form-urlencoded\r\n";
$header .= "Content-Length: " . strlen($req) ."\r\n\r\n";
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);