The information in your query is somewhat ambiguous, but based on my interpretation, you are seeking a method to verify any value of $page against a link value stored in a database table (pages?) without manually listing all potential values in a switch statement.
If I have understood correctly, the following rough function can assist you with this task. In a live high-traffic setting, it would be essential to introduce caching mechanisms to avoid excessive database queries on each page load, along with robust input validation - both aspects that are not encompassed in the provided demonstration. Nevertheless, this should offer some direction for your next steps:
Common library file:
/**
* Given a page name, this function checks if there is an associated
* link in the database.
* If found, return the link value, otherwise return false.
*/
function getTemplate($page)
{
// Query the database to check for a link corresponding to this page
// For busy sites, consider implementing caching strategies
$query = sprintf("SELECT link FROM pages WHERE name = '%s'",
mysql_real_escape_string($page));
$result = mysql_query($query, $db_cnx);
// Any results obtained?
if (mysql_num_rows($result) > 0)
{
// Assumption: 'name' is unique in the db
$row = mysql_fetch_assoc($result);
return $row['link'];
}
else
{
return false;
}
}
Header.php:
include('common.lib.php');
if(isset($HTTP_GET_VARS['mod']))
{
$page = $HTTP_GET_VARS['mod'];
}
else
{
$page = 'home';
}
// Verify if our page is linked in the database
$template = get_template($page);
if($template)
{
require( $template );
}
else
{
// No corresponding link was found in the database
echo('Error: There is no file on this server with that name');
}