I’m not an ASP guy but i think PHP is more portable and ASP relies on being on Windows server rather than Unix whereas PHP can run on any.
The reasons to go PHP I would say, from your angle, would be to make your site easier to update (ie: files of daa that you only need change once to affect all pages) like links panels, headers and footers etc and “dynamism” – loading content based on certain criteria fed by parameters (ie: searches etc).
You can get basic scripts for free to do a lot of things and learn from these. If you intend being in the game for the long haul and envisage building a large site, its worth it. If you just think 20 pages of content or so, then maybe HTML is fine. But HTML will be harder to scale in the longer term.
Cheers
Simsi