Custom Session Handlers in PHP4 Page 3

A DBM Session Handler

Our first example is to write a customized session handler to save session
data into a DBM file. (This is the session_dbm.php file from ying20000602.zip.)
There are many reasons why you might want to do this,
for example, if you were on a shared server from your ISP and you did not
want your sessions mixing with those from their other client’s scripts.

IMPORTANT NOTE:

You must have DBM support in your PHP4 before you try this.
If you do not things can get ugly, real ugly!

The approach we are going to take is to have one DBM file that stores
all the session data (in case you don’t know, a DBM file is like a very
simple database that only holds key/value pairs). To fit this into the
6 functions:

1. sess_open($sess_path, $session_name);
   We will open the DBM file in read/write mode by calling
   dbmopen(). The name of our DBM file will be /tmp/PHPSESSID
   unless you’ve modified the session path and name settinsg
   in your php.ini file.
2. sess_close();
   In this function, we are simply going to close the DBM file
   properly by calling dbmclose().
3. sess_read($key);
   Here we just call dbmfetch() to load up the session data associated
   with the session key. When loading up a session, it is necessary
   to make sure I’m not reading something that’s been expired, so we will
   attach a timestamp to the session.
Why? So that incase it has expired, but hasn’t been removed out for
whatever reason, we don’t accidentally read in expired data. This
would be a big no-no!
We know that DBM only stores key/value pairs, so we have to glue
the timestamp onto the value when writing the session data, and extract
it when reading in the session data. Any session who’s timestamp has
been expired will be ignored. See the source code, it will make more
sense there.
4. sess_write($key, $val);
   To write a session, we will use the dbmreplace() function. Note
   from above that we want to save an expiry timestamp on the session,
   so we will attach it to the value.
5. sess_destroy($key);
   Destroying a session is easy, we just call dbmdelete() to remove it
   from the session file.
6. sess_gc($maxlifetime);
   Garbage collection is a bit nasty and intensive here, what we must do
   is loop through all the sessions that we’ve saved in our DBM file and
   delete the ones that have expired. This can be slow because we must
   loop through all the sessions stored in the file.

So now we’ve got a DBM session handler, cool! Now let’s get these
sessions stored in a MySQL database.