PHP Sessions

The session_start( ) function is used to create a new session. A session is unique to the interaction between a browser and a web database application. If you use your browser to access several sites at once, you'll have several unrelated sessions. Similarly, if several users access your application each has their own session. However, if you access an application using two browsers (or two browser windows) at the same time, in most cases the browsers will share the same session; this can lead to unpredictable behavior—that's the reason why many web sites warn against it.

The first time a user requests a script that calls session_start( ), PHP generates a new session ID and creates an empty file to store session variables. PHP also sends a cookie back to the browser that contains the session ID. However, because the cookie is sent as part of the HTTP headers in the response to the browser, you need to call session_start( ) before any other output is generated

The session identifier generated by PHP is a random string of 32 hexadecimal digits, such as fcc17f071bca9bf7f85ca281094390b4. When a new session is started, PHP creates a session file, using the session identifier, prefixed with sess_, for the filename. For example, the filename associated with our example session ID on a Unix system is /tmp/sess_fcc17f071bca9bf7f85ca281094390b4.

Using Session Variables

The session_start( ) function is also used to find an existing session. If a call is made to session_start( ), and a session has previously been started, PHP attempts to find the session file and initialize the session variables. PHP does this automatically by looking for the session cookie in the browser request whenever you call session_start( ). You don't need to do anything different when starting a new session or restoring an existing one. Even if the identified session file can't be found, session_start( ) simply creates a new session file.

A simple PHP script that uses a session

<?php

  require_once "HTML/Template/ITX.php";



  // This call either creates a new session or finds an existing one.

  session_start( );



  // Check if the value for "count" exists in the session store

  // If not, set a value for "count" and "start"

  if (!isset($_SESSION["count"]))

  {

    $_SESSION["count"] = 0;

    $_SESSION["start"] = time( );

  }



  // Increment the count

  $_SESSION["count"]++;



  $template = new HTML_Template_ITX("./templates");

  $template->loadTemplatefile("example.10-2.tpl", true, true);



  $template->setVariable("SESSION", session_id( ));

  $template->setVariable("COUNT", $_SESSION["count"]);

  $template->setVariable("START", $_SESSION["start"]);

  $duration = time( ) - $_SESSION["start"];

  $template->setVariable("DURATION", $duration);



  $template->parseCurrentBlock( );



  $template->show( );

?>