Accessing html parameter in PHP

<?php

if ($_POST['destroy'])
{
    session_destroy();
}
else
{
    session_start();
}

?>

<html>
<head>
<title>Session test</title>
</head>

<body>

<?php

if (isset($_POST['destroy']))
{
    echo "Destroy set";
}

$_SESSION['counter']++;

echo "You have visited this page " . $_SESSION['counter'] . " times" . "<BR>";
echo "I am tracking you using the session id " . session_id() . "<BR>";
echo "Click <a href="" . $_SERVER['PHP_SELF'] . "?destroy=1">here</a> to destroy the session.";
?>

$_POST['destroy']

$_GET['destroy']

session_start();
$_SESSION = array();
if (isset($_COOKIE[session_name()])) {
    setcookie(session_name(), '', time()-42000, '/');
}
session_destroy();

if( $_GET['destroy'] == 1 )

if( isset($_GET['destroy']) )