CMS & WordPress
is it possible to convert a php - mysql site into wordpress? How?
Wordpress uses PHP and MySQL so I would say yes. Though any site can be converted to use WP. How to go about doing it will be harder to answer. How familiar are you with Wordpress, Wordpress Themes?
In short: Yes
Longer answer: Yes, but it depends.
Building a Wordpress site is trivial, writing themes isn't all that hard, there are of course some Wordpress functions and templating things that you should know about, but for the most part you can rely on the Wordpress Codex for that.
If you have a lot of custom data collection happening you could either write a Wordpress Plugin that does the same, or you could write them as a webservice, of course you could also keep them as standalone pages. I guess it all comes back to the "it depends".
If your site has a lot of content e.g. news articles, blog article etc., you could probably write an RSS feed or some such to import content in to Wordpress.
Like Retro said, it will probably also depend on how adept you are at WP
That might make it a little harder, but still, depending on your site, it could be quite easy to convert.
Is the site you currently have a big site with a lot of content? What's the reason you want to move to Wordpress?
Then you'll need a WP developer. It's faster, cheaper, better and more secure to hire an experienced developer instead of trying and fiddling yourself.
You could if you would, but you'll need to learn WP's template tags and -at least- know how The Loop works.
If you are not familiar with wp, I would suggest you to hire a designer and get your current template convert to a wp theme. After that, just copy the content over =) Hope this helps.