Displaying other document extensions in a PHP/mysql database - php

Question

OK this is my first post here so, hello! and please go easy on me if this is a simple solution.
I am creating a document database using apache2,mysql and php. I have only had previous experience with html,css and a small amount of javascript so this has been a steep learning curve and a good challenge for me.
I have successfully created the database and uploaded documents into it. i can search and link to the documentment required in a html/php screen but i only seem to be able to use/search, recall one extension type. this would be either .PHP, .PDF, .html, or .docx etc. In reality not all documents are or will be PDF's, below is my script:
/* -- end of connection script --*/
if(isset($_GET['search'])) {
$get_value = $_GET['user_query'];
if($get_value=='') {
echo "<center id='alert'><b>nothing in the search field</b></center>";
exit();
}
if($get_value=='Search...'){
header('Location: index.php');
}
$result_query = "SELECT * from search WHERE site_keywords LIKE '%$get_value%' OR site_title LIKE '%$get_value%' OR site_description LIKE '%$get_value%' ";
$run_result = mysql_query($result_query);
?>
<h3 class="total"><?php echo $row_result; ?>Results for '<?php echo $get_value; ?>'</h3>
<?php
if(mysql_num_rows($run_result)<1) {
echo "<center id='alert'><b>Oops! no search results found</b></center>";
exit();
}
while($row_result=mysql_fetch_array($run_result)) {
$site_id=$row_result['site_id'];
$site_title=$row_result['site_title'];
$site_link=$row_result['site_link'];
$site_description=$row_result['site_description'];
$site_file=$row_result['site_file'];
echo"<div class='results'>
<h3><a href='$site_id.php' target='_blank'>$site_title</a></h3>
<p align='justify'>$site_description</p>
</div>";
}
}
?>
as you can see from the echo results the href file requires an extension (in this case .php) to correctly display a .php document but not other extension types. If i change this to .pdf it will display the .pdf but not .php and so on. I am at a loss.