Description:
------------
The strip_tags function is hardcoded to strip out <!...> tags, such as comments.

It is IMO expected behaviour for the function that specifying "<!-->" or "<!DOCTYPE>" in the allowed tags list leaves these intact.

If this is expected behaviour for this function, then the documentation should be updated.

(PS: The state machine in php_strip_tags() in string.c seems to contain some redundancies. Someone who understand all of it should take a closer look ;))

Reproduce code:
---------------
<?php
  print strip_tags("Some text. <!-- a comment --> Some text.", "<!-->");
?>

Expected result:
----------------
The output should be:
Some text. <!-- a comment --> Some text.
(with the comment intact)

Actual result:
--------------
The output is:
Some text.  Some text.
(the comment is stripped out)