Sessions allow variables to be stored on the server so the variables can be restored each time a user requests a script. Consider a short example:

<?php
  session_start( );
          
  if (session_is_registered("count"))
  {       
     echo "Hello! You've visited {$count} times";  
     $count++;
  }       
  else    
  {
     echo "Welcome new user!";
     session_register("count");
     $count = 1;
  }
?>

When the user requests the script for the first time, a new session is created. Then, a variable $count is registered and stored on the web server with its associated value of 1. When the script is requested again, the variable is automatically restored by the PHP engine and the count incremented. For example, on the fifth request of the script the output is:

Hello! You've visited 5 times

<?php
  // Log out of the system
  
  session_start( );
  session_destroy( );
  
  // Redirect to the confirmation page.
  header("Location: logout.html");
?>

// Redirect to the confirmation page.
header("Location: logout.html");

The Location: header instructs the web browser to immediately request another resource; in this case, the logout.html page. Therefore, when the user clicks on the link to log out, the logout.php script destroys the session, and the logout.html page is displayed. We use this redirection so that if the user reloads or refreshes the logout.html page, no unnecessary session activity occurs. The page thanks the user for using the application.

NOTE: The header( ) function causes a very common error in which the PHP engine complains that it cannot add header information because the headers have already been sent.

The error occurs because the web server sends headers as soon as any HTML is output. If you leave a blank line or even a single space before the PHP start tag, the headers are sent, because these are treated as HTML (albeit not very interesting HTML).

function logincheck( )
{
   session_start( );
  
   if (!session_is_registered("user"))
      // redirect to the login page
      header("Location: index.php");
}