Nice to see another PHP scripter out there :L lacks a few on this site hehe.
I must be bored or something.. but this would incorporate all your HTML & PHP together;
<?php
echo '<html>
<body>';
if (!$_POST) {
echo '<form action="' .$_SERVER["PHP_SELF"]. '" method="post">
Age: <input type="text" name="age" />
<input type="submit" />
</form>';
}
else {
$age = $_POST["age"];
if ($age < 18) {
echo 'You are too young';
}
else {
echo 'Welcome';
}
}
?>