12.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>
<!--Write your PHP Script here-->
<?php
if(isset($_GET['num'])) {
    $num = intval($_GET['num']);
    $a = 1;
    $b = 1;
    $c = 2;
    echo $a . " " . $b . " " . $c . " " . " ";
    for($i = 3; $i < $num; $i++) {
        $d = $a + $b + $c;
        $a = $b;
        $b = $c;
        $c = $d;
        echo $d . " ";
    }
}
?>
</body>
</html>