Having created a session and saved session data for use across iterations of
pages within a session, we can see that there would be one more very useful
capability. What if we could save session information from one session to
another, returning to information that was saved perhaps a few days ago?
PHP provides this capability by enabling you to save session information in a
file.
A particularly useful example of the application of this ability, is to
capture and refer to username and password information. You could, for
example, create a login page where a visitor can enter their user Id (their
email address, perhaps) and their password, You would then retrieve
session information from an earlier session, based on their user Id, and present
them with customized views of the rest of your site, based on other stored
session variables that you retrieve from the file. Such a project would,
of course, require a little more code that is shown in this tutorial, but the
methods used to save and retrieve the session data are right here!
Another great example of the use of this feature is in conjunction with a
shopping cart. Suppose you have a shopping cart that builds an order as
the user makes their selections and stores that information somewhere based on a
unique identifier such as a combination of their user Id and the session number.
Now suppose their connection is interrupted and their session is lost. If
you had saved their original session information in a file, then when they
reconnected and logged back into your site, you would have all the information
you would need to retrieve their partially completed order. That would
make for a happy shopper!
And so to the technique, which is simple enough: we’re going to use the fopen
function to open our file, the session_encode function to encode our session
information into a string, the fputs function to write it into our file and the
fclose function to close the file. Here’s an example:
<?php
session_register("username");
session_register("password");
session_register("ordernumber");
$username = "Goody";
$password = "mypass";
$ordernumber = "1234";
$sessionfile = fopen("sessionfile.txt", "w");
fputs($sessionfile, session_encode( ) );
fclose($sessionfile);
?>
As you can see, we are creating three session variables to hold the
information we will need and, in this example, are simply populating those
variables from text strings. you will remember from the
file system tutorial in this series about file use of the file functions
shown here. You can see here that in the fputs instruction we are using
the session_encode function to take all our session information (which includes
all our session variables, but does not include the session number) and encode
it into a string which becomes the information we write (fputs) to our file.
Now that we have our information safely tucked away, we need a method to
retrieve it again when we need it. For this, we’ll use the fgets function
to get the record and the session_decode to retrieve the session information
from the record returned. Here we go:
<?php
session_start( );
?>
<html><head> ……. etc.
<?php
$sessionfile = fopen("sessionfile.txt", "r");
session_decode(fputs($sessionfile, 4096) );
fclose($sessionfile);
?>
There are a couple of things to notice here. First, you’ll see that
we started a new session (with the session_start coming before anything else, as
required – remember that
from here?) because we need to have an active session to be able to create
the session_variables, which is what the session_decode is going to do.
Next you’ll notice that the session_encode and session_decode instructions
are written the other way round from each other, with respect to their
associated fputs and fgets functions. Think of it like this: when writing,
we want to write the result of the session_encode so the fputs contains the
session_encode; when reading, we want to decode the result of the fgets
function, so the session_decode contains the fgets. (Technically, it may
be inaccurate to speak of one "containing" the other, but it certainly helps to
think of it that way!)
Another thing to watch
out for is the scope of our variables (remember scope form the
functions tutorial?) In the first example above, we have specific
statements to define the session variable, so their scope may be more obvious,
but in the second, the variable will be defined by the session_decode function
and so the scope might not immediately occur to you. In either case, if
the definition of the variable occurs within a function (that is, you have
written these instructions inside some function) the scope of the
session_variables will be local to that function. If that’s not what you
want, you would have to add the "global" parameter. In the second example
above, this would mean adding a global definition for the variables prior to
invoking the session_decode function, like this:
$sessionfile =
fopen("sessionfile.txt", "r");
global $username, $password, $ordernumber;
session_decode(fputs($sessionfile, 4096) );
fclose($sessionfile);
Finally, you might want
to think a little about the file name you use. In these examples we used a
file called "sessionfile.txt". It might be more useful to you to name your
file something else, such as the user Id, or a combination such as the user Id
and an application identifier (for example, "vincebarnesorders.txt") so that you
will know which file to get your session information from when the user comes
back into your page.
