Confused how to make a twig function

I want to

Output some PHP code result in a Twig template

I made a new plugin with those three files but I get the following error
Declaration of Grav\Plugin\ExampleTwigExtension::getFunctions() must be compatible with Grav\Common\Twig\Extension\GravExtension::getFunctions(): array

I have seen this
load the Twig extension from your theme/plugin

Does that mean I should put the plugin into user/themes/quark ?