add_action('init', 'do_rewrite');
function do_rewrite(){
add_rewrite_tag('%play%','([^&]+)');
add_rewrite_rule('play/super-mario-bros.html$','index.php?p=1182&play=true','top');
}
<body>
<?php
global $wp_query;
if ($wp_query->query_vars['play']) echo 'query_vars='.$wp_query->query_vars['play'];
?>
if (isset($_GET['play'])) add_action('init', 'do_rewrite');
function do_rewrite(){
add_rewrite_tag('%play%', '([^&]+)');
add_rewrite_rule('^/play/?p=([1-9]+)?', 'index.php?p=$matches[1]&play=$matches[2]', 'top');