Answer 1

If it were me and I were using a paragraph to welcome the user, for example, I would use:
<p>Welcome, <?pho echo $_SESSION['username']; ?>.</p>
Basically, as long as you use proper php syntax, all you have to do is treat it like any other word in whatever element it is in.
Please note: the variable name can be changed! I just used that example because I have coded that myself.