Abbas | 
09 28th, 2007| 
On many banking sites you will find that when back or refresh button is pressed the session expires and it shows you a custom page. Well below is a simple PHP code which will do the same. The code is very basic and illustrates just the logic.
Include the below given code in all your scripts.
PHP:
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// First make sure that the page is not cached
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// Start the session
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// If there is a key in session and there is no key in GET then exit with error.
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echo "SESSION expired";
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exit;
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}
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// If there is a key in GET then validate the key against the key stored in session
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// Compare the key passed with the one stored in session
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if ($_GET['key'] != $_SESSION['key']) {
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echo "SESSION expired";exit;
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}
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}
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// Generate a key for next page and store it in session.
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$_SESSION['key'] = $next_key;
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// Now $next_key is the key which should be passed in all links
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// Something like <a href="second_page.php?key=$next_key">Secon page</a>
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// If no key is passed or invalid key is passed then session expire error will be shown
$next_key should be passed in the URL to all scripts.