How do I use a Codeigniter method within my own function?

I use Codeigniter as my PHP framework, and in one of my controllers, I want to include a Codeingiter method within a function I created myself. The CodeIgniter method that I want to include in my function is "$this->db->escape($var);", which is an escaping query for Codeigniter. My PHP code is the following:

foreach ($array as $item)
{
  $name = $item["name"];
  $name = processvar($name);
}

function processvar($var) {
  $var = $this->db->escape($var);
  return $var;
}

However, the code above produces the following error message:

Fatal error: Using $this when not in object context in [folder path]/index.php on line #

Please advise how I can include the "$this->db->escape()" method in my own custom function.