(PECL eio >= 0.0.1dev)
eio_sync_file_range — Sync a file segment with disk
$fd
   , int $offset
   , int $nbytes
   , int $flags
   [, int $pri = EIO_PRI_DEFAULT
   [, callable $callback = NULL
   [, mixed $data = NULL
  ]]] )
   eio_sync_file_range() permits fine control when synchronizing the open file referred to by the file
 descriptor fd with disk.
  
fdFile descriptor
offsetThe starting byte of the file range to be synchronized
nbytes
     Specifies the length of the range to be synchronized, in bytes. If
     nbytes is zero, then all bytes from offset through
     to the end of file are synchronized.
     
flags
     A bit-mask. Can include any of the following values:
     EIO_SYNC_FILE_RANGE_WAIT_BEFORE,
     EIO_SYNC_FILE_RANGE_WRITE,
     EIO_SYNC_FILE_RANGE_WAIT_AFTER. These flags have
     the same meaning as their SYNC_FILE_RANGE_*
     counterparts(see SYNC_FILE_RANGE(2) man page).
     
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_sync_file_range() returns request resource on success or FALSE on error.