Are you sure you've tested this?
Just by looking at it, I see the following problems:
1) I haven't found the PG_* constants, which you use in __costruct(), in the documentation, so I think they are user-defined. Thus I would suggest making them class constants
2) _open() uses the property "post", which is nowhere defined (or I'm just blind).
3) Instead of using:
if($abc === false)
you could simply do:
// or if you want to invert it
4) _read() may return ' ' (note the space). Is this behavior desired or should it return an empty string? The documentation says that the read function should return an empty string, if there's nothing to read (and that's the case here, imho).
5) The SQL in _read() is messed up, too. There are two single quotes, which should be escaped, but aren't.
6) In _write() $results is sometimes misspelled as $reults or $reulsts.
7) I think the insert query in _write() is never reached (if you fix the misspellings noted as #6).
8) Is it necessary that the sess_id column is 255 chararcters long? I thought that a session id would always be an 40-hexadecimal-character string.