Session variables

For the following code blocks:

#---------- top.php              first program entered

session_start(); session_register ('trackingnumber'); if (IsSet($trackingnumber)){ print ("Issued session destroy because trackingnumber = $trackingnumber<br>"); session_destroy(); print ("trackingnumber = $trackingnumber<br>"); } else { print ("trackingnumber has not been set<br><br>"); } print ("<html>"); print ("<head>"); print ("</head>"); print ("<body>"); print ("<h3>top.php is the entry point ...</h3>"); print ("<h3>This is the value of trackingnumber: $trackingnumber<br><br>"); print ("<a href=\"middle.php\">To middle</a>"); print ("</body>"); print ("</html>");

#---------- middle.php                 second program entered

session_start(); session_register('trackingnumber'); if (!IsSet($trackingnumber)){ $mtime = explode (" ",microtime()); $mseconds = $mtime[0] * 1000000000; srand($mseconds); $trackingnumber = rand(); print ("<h3>tracking number on input is: ".$trackingnumber."<br></h3>"); print ("<h3>was reset<br><br></h3>"); } else { print ("<h3>tracking number on input is: ".$trackingnumber."<br></h3>"); print ("<h3>was not reset<br><br></h3>"); } $rnumber = 1234567; print ("trackingnumber: $trackingnumber<br>");

print ("rnumber:         $rnumber<br><br>");

print ("session name: ".session_name()); print ("<br>"); print ("session id: ".session_id()); print ("<br>"); #----- check for the previous session's existance print ("<html>"); print ("<head>"); print ("</head>"); print ("<body>"); print ("<h3>middle.php<br><br>This is the session id being passed to bottom: ["); print ("$trackingnumber"); print ("]</h3>"); print ("<a href=\"bottom.php?Session=$trackingnumber\">To bottom<br><br></a>"); print ("<a href=\"top.php\">To top</a>"); print ("</body>"); print ("</html>");

#---------- bottom.php         third program entered

session_start(); parse_str($QUERY_STRING); print $Session; print ("<html>"); print ("<head>"); print ("</head>"); print ("<body>"); print ("<h3>bottom.php<br><br>This is what was passed from middle: ["); print $Session; print ("]</h3>"); print ("<a href=\"middle.php\">Back to middle</a>"); print ("</body>"); print ("</html>"); Entering program top.php, I check whether the variable $trackingnumber has been set. If so, I want to destroy its contents. $trackingnumber should be empty upon entry to middle.php, where it gets set and passed to bottom.php. In traversing backwards, entry to top.php should empty the contents of $trackingnumber. Why doesn't the session_destroy() perform that? Bob