PHP Define var = one or other (aka: $var=($a||$b);)

Yes you need to use simple ternary operator which you have used within your example along with some other functions of PHP like as of isset or empty functions of PHP. So your variable $x will be assigned values respectively

Example

$x = (!empty($_POST['x'])) ? $_POST['x'] : (!empty($_SESSION['x'])) ? $_SESSION['x'] : NULL;

So the above function depicts that if your $_POST['x'] is set than the value of

$x = $_POST['x'];

else it'll check for the next value of $_SESSION if its set then the value of $x will be

$x = $_SESSION['x'];

else the final value'll be

$x = null;// you can set your predefined value instead of null

$x = (!empty($a)) ? $a : (!empty($b)) ? $b : (!empty($c)) ? $c : (!empty($d)) ? $d : null;

If you need a function then you can simply achieve it as

$a = '';
$b = 'hello';
$c = '';
$d = 'post';

function getX(){
    $args = func_get_args();
    $counter = 1;
    return current(array_filter($args,function($c) use (&$counter){ if(!empty($c) && $counter++ == 1){return $c;} }));
}

$x = getX($a, $b, $c, $d);
echo $x;

Update

I've managed to create a function for you that achieves exactly what you desire, allowing infinite arguements being supplied and fetching as you desire:

function _vars() {
    $args = func_get_args();
    // loop through until we find one that isn't empty
    foreach($args as &$item) {
        // if empty
        if(empty($item)) {
            // remove the item from the array
            unset($item);
        } else {
            // return the first found item that exists
            return $item;
        }
    }
    // return false if nothing found    
    return false;
}

^{To understand the function above, simply read the comments above.}

Usage:

$a = _vars($_POST['x'], $_SESSION['x']);

And here is your:

Example

---

It is a very simply ternary operation. You simply need to check the post first and then check the session after:

$a = (isset($_POST['x']) && !empty($_POST['x']) ? 
        $_POST['x']
        :
        (isset($_SESSION['x']) && !empty($_SESSION['x']) ? $_SESSION['x'] : null)
    );