missing a few html link basics

Hi all,

I seem to be missing a few html link basics. What is the significance of:

starting slash...?

<form action="/mypage.php" method="POST">  like:

I know these can redirect:

dig down..

/myfolder/mypage.php

and up

../myfolder/mypage.php

but now I am seeing a starting slash and a page ???

/mypage.php

or

./mypage.php

instead of just = mypage.php

Q: why use starting ./ or / instead of just the page?

MORE...

and while I am at it... I always use closing php tag like:

<?php

stuff...

?>

... but now I am hearing if there is only php on the page - then it's best to *NOT* use a closing tag?

Q: Why is that?