I've been learning web development and the book I am learning from is using php tags within html files but they don't work for me, here is my header page:

```
<!DOCTYPE html>

<html lang="en">
<head>
    <meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />
    <title><?php echo $page_title; ?></title>
    <style type="text/css" media="all">@import "./includes/layout.css";</style>
</head>
<body>
    <a href="index.php" class="logo"> <img src="Includes/Images/NameIconSplash.png" /></a>
    <div id="wrapper">
        <div id="content">
            <div id="news">
                <div id="nav">
                    <ul>
                        // Tags don't work here
                        <?php
                            if ( (isset($_COOKIE['user_id'])) && (!strpos($_SERVER['PHP_SELF'], 'logout.php')) ) 
                            {
                                echo '<li><a href="logout.php">                                           Logout</a></li>'
                            } 
                            else 
                            {
                                echo '<li><a href="login.php">                                            Login</a></li>'
                                echo '<li><a href="register.php">                                         Register</a></li>'
                            }
                        ?>

                    </ul>
                </div>

```

The php code is just normal black text but it recognises the html code within the php tags. Does someone know why it doesn't recognise them?

Thankyou in advance.