Tribonacci Sequence

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>First Steps Into PHP</title>

</head>
<body>
<form>
    N: <input type="text" name="num" />
    <input type="submit" />
</form>
<?php
if (isset($_GET['num'])) {
    $num = intval($_GET['num']);
    if ($num == 1) {
        echo "1";
    }
    else {
        $fib = 0;
        $set = [1, 1, 2];
        for ($x = 3; $x < $num; $x++) {
            $set[$x] = $set[$x-1] + $set[$x-2] + $set[$x-3];
        }
        echo implode(" ", $set);
    }
}
?>
</body>
</html>