How to use $this in closure in php

$this is always available in (non-static) closures since PHP 5.4, no need to use it.

class Service {
    function delete_user($username) {   
        ...
        $sessions = $this->config->sessions;
        $this->config->sessions = array_filter($sessions, function($session) {
            return $this->get_username($session->token) != $username;
        });
    }
}

See PHP manual - Anonymous functions - Automatic binding of $this