You can define the extension so that it needs the environment. Twig will automatically pass it to the function.

use Twig\Environment;use Twig\TwigFunction;public function getFunctions(){    return [        new TwigFunction(            'myfunction',            [$this, 'myFunction'],            ['needs_environment' => true]        ),    ];}public function myFunction(Environment $environment, string $someParam){    // ...}

For older versions of Twig

public function getFunctions(){    return array(        new \Twig_SimpleFunction(            'myfunction',            array($this, 'myFunction'),            array('needs_environment' => true)        ),    );}public function myFunction(\Twig_Environment $environment, string $someParam){    // ...}

Using this function the user can pass the twig environment instance to a twig extension

private $environment;public function initRuntime(\Twig_Environment $environment){    $this->environment = $environment;}

@tvlooy answer give me a hint but didn't work for me. What I needed to to to achieve it is:

namespace AppBundle\Twig;class MenuExtension extends \Twig_Extension{    public function getName()    {        return 'menu_extension';    }    public function getFunctions()    {       return [           new \Twig_SimpleFunction('myMenu', [$this, 'myMenu'], [               'needs_environment' => true,               'is_safe' => ['html']           ])       ];    }    public function myMenu(\Twig_Environment $environment)    {          return $environment->render('AppBundle:Menu:main-menu.html.twig');    }}

I needed addtionaly add 'is_safe' => ['html'] to avoid autoescaping of HTML.

I've also registered the class as symfony service:

app.twig.menu_extension:    class: AppBundle\Twig\MenuExtension    public: false    tags:      - { name: twig.extension }

in TWIG template I've added {{ myMenu() }}

I work with "twig/twig": "~1.10"and Symfony 3.1.3 version