Numbers from 1 to N

<!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'])) {
    $set = [];
    $n = intval($_GET['num']);
    for ($x = 1; $x <= $n; $x++) {
        $set[$x] = $x;
    }
    echo implode(" ", $set);
}
?>
</body>
</html>