(PECL eio >= 0.0.1dev)
eio_sendfile — Transfer data between file descriptors
$out_fd
   , int $in_fd
   , int $offset
   , int $length
   [, int $pri
   [, callable $callback
   [, string $data
  ]]] )eio_sendfile() copies data between one file descriptor and another. See SENDFILE(2) man page for details.
out_fd
    Output file descriptor. Should be opened for writing.
in_fd
    Input file descriptor. Should be opened for reading.
offset
    Offset within the source file.
length
    Number of bytes to copy.
pri
    请求的优先级: EIO_PRI_DEFAULT, EIO_PRI_MIN, EIO_PRI_MAX, or NULL.
如果是 NULL , pri 将设为
EIO_PRI_DEFAULT.
callback
    
callback 函数将在请求结束时被调用. 
必须符合此原型: 
void callback($data, $result);
$data 是传给该请求的特定数据.
$result 包含特定请求的结果值, 通常是相应的系统调用的返回值.
    data
    
     Arbitrary variable passed to callback.
     
   eio_sendfile() returns request resource on success or FALSE on error.