Description:
------------
When strip_tags is used with a whitelist of tags, php allows slashes ("/") that occur inside the name of a whitelisted tag and copies them to the result.

For example, if <strong> is whitelisted, then a tag <s/trong> is also kept.

The browsers Chrome, Firefox and Safari, however, interpret this syntax as <s trong=""> (in HTML this would result in a strikethrough element with an unknown attribute). This means that it's possible to use any tag which is a prefix of a tag that is whitelisted. If the whitelist is important for security then this can allow the introduction of non-whitelisted tags.

Test script:
---------------
<?php

echo strip_tags("<s/trong>b</strong>", "<strong>");

Expected result:
----------------
b

Actual result:
--------------
<s/trong>b</strong>