Just use (title, body) VALUES ('$title', '$body'). You don't have to specify empty strings for fields you're not using or want to auto increment.

$_POST is the longhand. Just use the name of the form element as a variable. If you were using it, then you'd probably want to do '$_POST[title]' (or maybe "$_POST['title']"?) as you can't have single quotes within single quotes.