CodeIgniter - Not allowing $this usage within Function

Trying to show a category tree (with subcats and sub-subcats)

Using this code:

<?php 
	$categories = get_categories();
	
	foreach($categories as $category) {
		    // Print the top level category
		    echo $category->title .'<br />';
		    
		    // get the sub categories
		    $subcategories = get_categories($category->id);
		    foreach($subcategories as $subcategory) {
		        echo '- '. $subcategory->title. '<br />'; 
		    }
		}
		
		function get_categories($parent = 0) {
		    $this->db->where('parent', $parent); 
		    $query = $this->db->get('bin_location');
		    return $query->result();
		}
	?>

PHP:

Getting: Fatal error: Call to a member function on a non-object in /home/domains/public_html/system/application/views/frontpage_view.php on line 25

Anyone have any ideas why I am having this problem?

 

You can't use $this-> in this context.
First you have to do:

$this->CI =& get_instance();

PHP:

then using $this->CI->db->where('parent', $parent) won't produce any error.