Strips HTML from inside PRE tags only

stripHTML

function stripHTML($str) {
    preg_match_all('#</?pre(>|\s[^>]*>)(.*?)</pre(>|\s[^>]*>)#is',$str,$matches);
    foreach($matches[2] as $match){
        $str =  str_replace($match,  htmlspecialchars($match),$str);
    }
    $str = preg_replace("/<\?/","&lt;?",$str);
    $str = preg_replace("/\?>/","?&gt;",$str);
    return $str;
}

Alternatively you can use strip_html() in place of htmlspecialchars() on line 5 if you want to completely axe the HTML rather than converting it into text. This seems a little useless in the pre tag, so I'm leaving it this way.
You can also replace both the pre's on line 2 with any other tag if you would like to modify the contents of a different tag.