Text selection, copy / paste are done in your browser client side, nothing to do with PHP which is server side. As for the weird behavior you're describing, it probably has more to do with the way the page is rendered in the browser, although I don't really understand / can't replicate it.
I've seen this before, but it has nothing to do with PHP. It has something to do with either improper nesting of your <p> tags, or something wrong with your CSS. I can't find the thread that covered this problem, but my suggestion to fix it is to validate both your html and css, and check for improper nesting.
(just saw Stretch's post, and that may be the problem; although I have seen this happen on site's that do not have any absolute positioning. Can anyone verify this with another source or thread?)
It is annoying but being new to PHP, I'm full of naive enthusiam and so I'll continue to upgrade from my frames and tables towards the nirvana that is csszengarden, which quite insired me yesterday. :)