Another option is to emit headers from PHP which allows IE etc to cache the search results page for a little while

e.g.

header('Pragma: ' . 'public');
header("Cache-Control: public");
header('Expires : ' . gmdate('D, d M Y H:i:s', time()+(60*10)) . ' GMT'

I had the "page expired" problem and this allows me to back button back to the PHP search results page....