Preview:
<?php
    if (isset($_POST['submit'])){
        $username = filter_input(INPUT_POST, "username", FILTER_SANITIZE_SPECIAL_CHARS);
        echo "Hello {$username}";
    }
?>

<!doctype html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <form action="index.php" method="post">
        <input type="text" name="username" id="username">
        <input type="submit" name="submit">
    </form>

</body>

</html>
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