For example:

PHP-code

<?
  header("Content-type: text/plain");
  echo strip_tags("the 5<6 but 5>4");
?>

will produce

the 54

I think, that strip_tags() "think", that "<6 but 5>" is normal HTML-tag, but this not true.

More examples:

echo strip_tags("start <32423423423> end");
echo strip_tags("start <????>not inside tag</????> end");

EXCLUSIVE example:
echo strip_tags('<IMG src="..." alt="next >>">');

It will produce
>">