The solution that should have been arrived at in this blog post: http://danielribeiro.org/solid-principles-violations-on-symfonys-cookbook-custom-twig-extension/

solution.php

<?php

interface MoneyFormatter
{
    public function format($value, $decimals=0, $decimalPoint='.', $thousandsSep=',');
}

class FormatterExtension extends \Twig_Extension
{
    private $formatter;
    private $filterName;

    public function __construct(MoneyFormatter $formater, $filterName)
    {
        $this->formatter = $formatter;
        $this->filterName = $filterName;
    }

    public function getFilters()
    {
        return array(
            new \Twig_SimpleFilter($this->filterName, array($this->formatter, 'format')),
        );
    }

    public function getName()
    {
        return "{$this->filterName}_formatter_extension";
    }
}