PHP Programming/Why Templating

So what is a template, in the first place? For our purpose, a template is an HTML-like document that defines the presentation of a web page, while a PHP script supplies the content. The separation of content and presentation is at the heart of any GUI paradigm. To see why this is desirable, consider the following hypothetical, yet realistic script. It's a script to retrieve a list of books from the database and display it as a table with alternate colours for each row, or "No books found" if the list is empty.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
       "http://www.w3.org/TR/html4/loose.dtd">
<html>
   <head>
      <title>List of Books</title>
   </head>

    <body>

<?php
$books = new Array();
$i = 0;

$database = open_database();
if (is_null($database)) {?>
    <h1>Error opening database</h1>
    Please contact the system administrator at alice@wonderland.com and report the problem.
    </body>
    </html>

<?php
    exit();
}

$result = $database->query("SELECT ISBN, title, author FROM book");
while ($row = $result->retrieveAssoc()) {
    $books[] = $row;
}?> 

    <table>
        <?php
        if (count($books) == 0) {
            echo "<tr><td>No books found</td></tr>";
        }
        else {
            foreach ($books as $book) {
                if ($i % 2 == 0) {
                    echo "<tr bgcolor='#EEEEEE'>";
                } else {
                    echo "<tr bgcolor='#FFFFFF'>";
                }
                $i++; ?>
                <td><?php echo $book['ISBN']; ?></td>
                <td><?php echo $book['title']; ?></td>
                <td><?php echo $book['author']; ?></td>
                </tr>
        <?php
            }
        }?>
    </table>
</body>
</html>

You will find yourself writing this kind of script very soon. The problems with this script are obvious:

This PHP page really is two pages, one for the error message and one for the normal display. Confusing.
Even for the normal display part, the PHP script does two things: prepare the data and format it, e.g. alternate colour for each row.
Look at how entangled the PHP and HTML codes are. Good indentation becomes practically impossible and that makes the code difficult to read.
What if your boss now wants not a table, but a list instead? You will need to alter your PHP code for that.
And now your boss wants a fancier error page that needs at least 300 lines of HTML, can you still tell where your first "page" ends and the second starts?
You probably need to teach your graphics designer PHP if he wants to do any aesthetic make up to your page.

The list goes on.

See also