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Oracle Tips by Burleson 


What are sessions and why are they used? As stated earlier, PHP is an interpreted scripting language and the interpreter is embedded into the web server processes. Below are processes that an Apache server spawns when the machine is started:

Each of those processes is able to interpret PHP scripts and it is unknown which among those processes will serve the next request. This means that all variables defined during the execution of a PHP script are “forgotten” as soon as another httpd process serves the next page. This applies to both global and static variables. Sessions answer the questions regarding how to make global variables really global.

HTTP (HyperText Transfer Protocol) is a stateless protocol, and no persistent connections are made to the web server. For easier programming and variable sharing, cookies and sessions are introduced.

Sessions and cookies are not specific to PHP. They are HTTP entities supported by all browsers and web servers. Cookies are entities carried in the headers of HTML pages and stored locally on the machines that use the browser to view pages served by a web server. Cookies contain content, expiration date and the name of the web server which requested the cookie be set. 

Below is a picture of the top portion of a cookie file for Mozilla browser on a Linux PC.

When a browser accesses a web server that has previously set a cookie on it, the browser sends to the server all valid cookies set by that server. This is an over simplification but it is sufficient for the purpose of this discussion. The full specification of the cookie mechanism can be found at:

Cookies are used to create sessions. HTTP is a stateless protocol. Stateless protocols do not allow permanent sessions. So what exactly are these sessions?

Sessions are objects that are maintained as persistent, (most frequently as files, but there are extensions that can put them in shared memory or a database) and which contain information that should be made persistent between requests. 

Sending that information over the network as a cookie for each request would be excruciatingly slow, so the variables needed for the script to work are kept in the session. How are sessions created?

Sessions are created and started by using the session_start()function. The session_start() function starts the session if it has  already been started and reads the session data if it has been started. Before going to more complicated examples, consider the example 7 from Chapter 1, now slightly reworked:

$ cat example7b.php
foreach ($N as $n) {
  if ($n=="next") { continue; }
  if ($n=="stop") { break;    }
  print "$n  ";
print "\n";
print "my session id is:$sess\n";
print "my session data is stored in /tmp/sess_$sess\n";

Functions session_start() and session_id() are being used, as well as the “superglobal” array $_SESSION. What happens when this script is executed?

$ ./example7b.php
1  2  3  4  5  6  7  8  9
my session id is:qdrsb53dpg5ju8t1i9e6a8sk82
my session data is stored in

Nothing special has happened. The loop is still executing as before, but there is new entity called “session”. Does the file shown by the script exist and what is in that file?

$ ls -l /tmp/sess_qdrsb53dpg5ju8t1i9e6a8sk82
-rw-------    1 mgogala  users         141 Oct 31 00:29 /tmp/sess_qdrsb53dpg5ju8t1i9e6a8sk82
$ cat /tmp/sess_qdrsb53dpg5ju8t1i9e6a8sk82

The file obviously exists and contains the array $N from the example, with the name “array_from_example7” as the key to the $_SESSION array. Any other script with the same session ID would have access to the same session file and would be able to read the variable from the $SESSION array as $_SESSION['array_from_example7']. 

How does a web server know the session_id of the session invoking the script? The session_start() function sets a cookie in the browser if a browser is used.  The cookie name is PHPSESSID and it can be easily found in the browser’s cookie file when present.

But how are things put into session? Here the story gets a bit complicated.  The best thing to use is $_SESSION built in global array. $_SESSION is accessible by a key which can be chosen as needed, but then must be used consistently across all the scripts interested in that particular session.

There is also a session_register() function Here is what the online manual returns for the session_register function:

See code depot for complete scripts

The above book excerpt is from:

Easy Oracle PHP

Create Dynamic Web Pages with Oracle Data

ISBN 0-9761573-0-6   

Mladen Gogala


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