At attempt to reproduce MDL-28161

test.php

<?php

require('config.php');
$CFG->enablehtmlpurifier = 1;

$PAGE->set_context(get_system_context());
$PAGE->set_url('/test.php');

echo $OUTPUT->header();

echo purify_html('A');

purge_all_caches();

echo purify_html('B');

echo $OUTPUT->footer();