见gateway.php文件中的注释：

// Oriental languages (Chinese, japanese, korean):
$gateway->setCharsetHandler( "none", "ISO-8859-1", "ISO-8859-1" );
$gateway->setCharsetHandler( "iconv", "big5", "big5" );
$gateway->setCharsetHandler( "iconv", "CP950", "CP950" );
$gateway->setCharsetHandler( "iconv", "Shift_JIS", "Shift_JIS" );
$gateway->setCharsetHandler( "iconv", "CP932", "CP932" );
$gateway->setCharsetHandler( "iconv", "CP949", "CP949" );

也就是说你的文件是用什么格式编码的，就把gateway.php文件中的$gateway->setCharsetHandler( "iconv", "big5", "big5" );中的两个“big5”改成相对应的编码名称就可以了。而在中国，一般使用“utf-8”、“gb2312”和“big5”，我用“utf-8”，所以在gateway.php文件中找到$gateway->setCharsetHandler("utf8_decode", "ISO-8859-1", "ISO-8859-1");一行代码，把它改成$gateway->setCharsetHandler("iconv", "UTF-8", "UTF-8");就能正常传递中文数据了！