Practically none... Maybe a bit of an extremely invalid HTML 4.01 and some javascript that worked only in internet exlorer

When I started making dynamic pages was when I also learned valid XHTML and more javascript.
With all due respect to TheSpecialist, I would not agree with him. In my opinion (which may be wrong as well), PHP is one of the easiest languages there are. To have any trouble with it, you must either come from a completely different language such as C++ or not read a manual before trying to write your scripts.