DELIMITER mydelimiter
CREATE FUNCTION `strip_tags`($str text) RETURNS text DETERMINISTIC
BEGIN
    DECLARE $start, $end INT DEFAULT 1;
    LOOP
        SET $start = LOCATE("<", $str, $start);
        IF (!$start) THEN RETURN $str; END IF;
        SET $end = LOCATE(">", $str, $start);
        IF (!$end) THEN SET $end = $start; END IF;
        SET $str = INSERT($str, $start, $end - $start + 1, "");
    END LOOP;
	RETURN $str;
END;
mydelimiter
DELIMITER ;


select strip_tags('<span>hel<b>lo <balise/> <a href="world">wo<>rld</a> <<x>again<.');
+--------------------------------------------------------------------------------+
| strip_tags('<span>hel<b>lo <balise/> <a href="world">wo<>rld</a> <<x>again<.') |
+--------------------------------------------------------------------------------+
| hello  world again.                                                            |
+--------------------------------------------------------------------------------+