What is the difference between these statements?

if(!isset($_POST["var"])){ }
if($_POST["var"]==""){ }
if(!$_POST["var"]){ }
My question is, what is the difference between each one?

I am using !isset($_POST["var"]) || $_POST["var"]=="" to see if they were not entered. I am doing this so that I can show an error, then return the form to the user.

BUT, I have a feeling I could just get rid of one of them.
Would the following two statements work the same?

if(!isset($_POST["var"]) || $_POST["var"]==""){
// Show error and form
}

if($_POST["var"]==""){
// Show error and form
}

in the context of checking for empty form fields, empty normally covers the bases...

<?if( empty( $_POST['var'] ) ){}?>