WordPress php function does not return value

I’m trying to write a PHP function for my WordPress theme that will return the number of categories for a given post. I’m using two files: functions.php and header.php

Contents of functions.php:

/**
*   Category count for a given post (post ID) excluding given excludecats (array).  
*   @return number
*/
function bl_cat_count($pid,$excludedcats) {
    $cat_count = 0;
    //$count = 0;
    $categories = get_the_category($pid);
    foreach($categories as $category) { 
        $cat_count++;
        if ( in_array($category->cat_ID, $excludedcats) ) {
            $cat_count--;
        } 
    }
    var_dump($cat_count);
    return $cat_count;
}

Contents of header.php:

$pid = $thumbnail->ID;
$excludedcats = array(1,85);
bl_cat_count($pid,$excludedcats);                          
var_dump($cat_count);
if ( $cat_count > 0 ) {
  // do something
}

var_dump on the function reveals the correct value. When calling from header.php however var_dump shows null. Maybe I’ve been looking at it for too long but I can’t figure out why.