Validate and submit multiple input fields in array in PHP: rank order

Question

I am using PHP to generate an HTML form for a ranking task, in order to obtain the rank order that the user assigns to a list of alternatives. The generated form is presented as

Answer 1

I figured out a simple solution that compares the $_POST array against a pre-specified array of accepted values, based on this.

The validation code:

$rankErr = "";
$hasErrors = false;

if ($_SERVER["REQUEST_METHOD"] == "POST") {
$rank = $_POST['rank_'];
$matchArray = array(1,2,3,4);
    if(is_array($rank) && is_array($matchArray) && array_diff($rank, $matchArray) === array_diff($matchArray, $rank)){
        $hasErrors = false;
    } else {
        $rankErr = "Please enter unique values (1-4) under Rank.";
        $hasErrors = true;
    }
    if (!$hasErrors) {
    include ("process.php"); 
    }
}

Seems to work perfectly. Hope this helps others with the same problem.