(PECL eio >= 0.0.1dev)
eio_truncate — Truncate a file
$path
   [, int $offset = 0
   [, int $pri = EIO_PRI_DEFAULT
   [, callable $callback = NULL
   [, mixed $data = NULL
  ]]]] )
  eio_truncate() causes the regular file named by path to be truncated to
  a size of precisely length bytes
  
path
    File path
offset
    Offset from beginning of the file.
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($data, $result);
$data is custom
data passed to the request.
$result contains
request-specific result value; basically, the value returned by corresponding
system call.
    data
    
     Arbitrary variable passed to callback.
     
   eio_busy() returns request resource on success or FALSE on error.