(PECL eio >= 0.0.1dev)
eio_fsync — Synchronize a file's in-core state with storage device
$fd
   [, int $pri = EIO_PRI_DEFAULT
   [, callable $callback = NULL
   [, mixed $data = NULL
  ]]] )Synchronize a file's in-core state with storage device
fd
    Stream, Socket resource, or numeric file descriptor.
pri
    The 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]);
data
is custom data passed to the request.
result
request-specific result value; basically, the value returned by corresponding system call.
req
is optional request resource which can be used with functions like eio_get_last_error()
data
    
     Arbitrary variable passed to callback.
     
    eio_fsync() returns request resource on success or FALSE on error.