Unexpected output on file include

[Mizutok]

I've been working on a custom CMS in PHP and haven't run into any problems until recently. When testing, I've noticed that the string \[code\]???\[/code\] has started appearing at the top of only the index page. Testing with some die statements throughout the code, it seems the output is coming between a file include.File A\[code\]<?phpif (!defined('IN_CMS')){ exit;}require(INCLUDE_PATH . '/pages/homepage/main.php');?>\[/code\]File B (Included File)\[code\]<?phpif (!defined('IN_CMS')){ exit;}$db->sql_query("SELECT * FROM `" . DB_PREFIX . "categories` active = 1");$cats = $db->sql_results();$categories = array();foreach($cats as $cat){ $cat_info = array( 'name' => $cat['name'], 'description' => $cat['description'], 'image' => $setting['site_url'] . '/thumbnails/' . $cat['image'], ); $categories[] = $cat_info;}include theme('index');?>\[/code\]I can't understand where the output is coming from, but it seems to be somewhere between the two files. I've added \[code\]ob_start()\[/code\] right before the include line in File A and \[code\]ob_end_clean()\[/code\] at the very beginning of the included file and the output does not show up, however there is no stray output anywhere in the two files.