PHP: a short cut for isset and !empty?

isset && !empty is redundant. The empty language construct is basically shorthand for !isset($foo) || !$foo, with !empty being equivalent to isset($foo) && $foo. So you can shorten your code by leaving out the isset check.

A much simpler way is:

$values = array('pg_title' => null, 'pg_subtitle' => null, …);
$values = array_merge($values, $_POST);

// use $values['pg_title'] etc.

If you don't want your default null values to be overwritten by falsey values, e.g. '', you can do something like this:

$values = array_merge($values, array_filter($_POST));

Just be aware that '0' is falsey as well.

You can use a simple function

function post_value_or($key, $default = NULL) {
    return isset($_POST[$key]) && !empty($_POST[$key]) ? $_POST[$key] : $default;
}

Then use:

$pg_title = post_value_or('pg_title');
// OR with a default
$pg_title = post_value_or('pg_title', 'No Title');

empty($var) is an abbreviation for !( isset($var) && $var ).

So !empty($_POST['...']) will be sufficient for your situation — the isset call you have currently is redundant.