How can I check if the last position of the array has been filled?

You can use the end() function to get the last element of the array and check whether it has value or not.

$arr = [1,2,3, null];

$ultimo = end($arr);

if($ultimo){
    echo 'tem valor: '. $ultimo;
}else{
    echo 'é vazio';
}

I would do so:

$array = [1, 2, 3];

$end = end($array);

if ($end) {
    echo "A última posição é {$end}";
}

Note: The end function only works with arrays stored in variables, since it expects a parameter passed by reference.

If you have problems with this, the way I usually solve this is to create a function that serves as a wrapper to be able to circumvent this "problem" in PHP:

function last(array $array) {
   return end($array);
}

last([1, 2, 3, 4, 5]); // int(5)

Another alternative is to get the index value, using count to get the total of elements and subtract by 1 :

$array = ['foo', 'bar', 'baz'];

if (($indice = count($array)) > 0) { // Se for um índice válido
    $ultimoValor = $array[$indice - 1];

    echo $ultimoValor;
}