How do I elegantly handle php content inclusion?

Question

I have a simple (excercise) website whose structure consist of a content selection menu and a content panel, where content is displayed when one the elements of the menu is selected. The menu use links of the type ?content_id=... to pass the information over.

I would like to handle 404 codes on inexistent ids or missing contents but to do so I have to compute the content of the content panel befor the page loads, so that I can output the correct header before everything else. I ended up writing this piece of code at the beginning of my file and printing the $content variable when necessary. Anyway, I feel quiet uncomfortable with this approach since I have to stop the natural flow to catch the include output.

<?php 
  // if no content is specified in the address, select the home page (0)
  $content_id = (isset($_GET['content_id'])) ? $_GET['content_id'] : 0;

  //check if selected content exists and put the content inside the $content variable (to be used later)
  if ($content_id < 0 OR $content_id > 2 OR !is_file('exContent/content'.$content_id.'.php')) {
    header("HTTP/1.0 404 Not Found");
    ob_start();
    include 'exContent/noContent.php';
  }
  else { 
    ob_start();
    include 'exContent/content'.$content_id.'.php';
  }

  $content = ob_get_contents(); ob_end_clean();
?>

I cannot figure out what's the best way to do this, I would be very happy if you can help me!

Answer 1

Could be a lot better but this is a start :)

  $includeFile 'exContent/content'.$content_id.'.php';

  if ($content_id < 0 OR $content_id > 2 OR !is_file( $includeFile )) {
    header("HTTP/1.0 404 Not Found");
    $includeFile = 'exContent/noContent.php';
  }

  ob_start()
  include $includeFile;
  $content = ob_get_contents();
  ob_end_clean();