(PECL eio >= 0.0.1dev)
eio_sendfile — Transfer data between file descriptors
$out_fd
   , mixed $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_fdOutput stream, Socket resource, or file descriptor. Should be opened for writing.
in_fdInput stream, Socket resource, or file descriptor. Should be opened for reading.
offsetOffset within the source file.
lengthNumber of bytes to copy.
priThe request priority: EIO_PRI_DEFAULT, EIO_PRI_MIN, EIO_PRI_MAX, or NULL.
If NULL passed, pri internally is set to
EIO_PRI_DEFAULT.
callback
callback function is called when the request is done.
It should match the following prototype: 
void callback(mixed $data, int $result[, resource $req]);
datais custom data passed to the request.
resultrequest-specific result value; basically, the value returned by corresponding system call.
reqis optional request resource which can be used with functions like eio_get_last_error()
data
     Arbitrary variable passed to callback.
     
   eio_sendfile() returns request resource on success or FALSE on error.