在外部PHP文件中回显ob_start回调

I'm working on population static HTML with database information in PHP. The HTML files have a pattern ({{$pattern}}) which when located in an HTML file will be replaced with a matching PHP variable. An example HTML file could be:

<div>{{$apples}}</div>

To populate this HTML file on demand, I've created the following PHP script (prepend.inc):

function ob_callback($buffer){

    global $data;
    $data['apples'] = CONSTANT; //global definition defined elsewhere

    $buffer = preg_replace_callback(
          '/{{\$([a-zA-Z0-9_]+)}}/',
          function($matches){
                 global $data;
             return (isset($data[$matches[1]])?$data[$matches[1]]:"");
          },
          $buffer);
    return $buffer;
}
ob_start("ob_callback");

This scrip is prepended to every HTML file by adding the following to the .htaccess in the HTML directory:

AddType application/x-httpd-php .html
php_value auto_prepend_file "prepend.inc"

Because of the architecture of my application, I require an HTML file echo'ed in PHP.

This has however proven to be a bigger task than expected because any attempt I make at converting the file to a PHP string and echo'ing results in the above ob_callback not being executed... (Althrough I know prepend.inc is run)

I have tried fread, file_get_content and even:

const CONSTANT = 'oranges';
ob_start();
require 'theHTMLfile.html';
$out = ob_get_contents();
ob_end_clean();
echo $out;

None of these however seem to have reached the ob_start callback in prepend.inc, and have had the pattern replaced...
One solution would be to use the full URL (require 'http://www.example.com/theHTMLfile.html';), but I CANNOT use that as it would break the reference to global variables, that I require to fetch data for the HTML etc..

Any suggestions would be much appreciated!