Here is the answer:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
</head>
<body>
<form>
N: <input type="text" name="num"/>
<input type="submit"/>
</form>
<?php
if (isset($_GET['num'])) {
$num = intval($_GET['num']);
for ($i = $num; $i >= 1; $i--) {
echo $i . " ";
}
}
?>
</body>
</html>