Would it be possible to add functions to Twig within the theme PHP file (IE: antimatter.php)?
Yes! In the class named after the theme inside a method onTwigExtensions, you can add something like:
// ...
   public function onTwigExtensions()
   {
       $function = new \Twig_SimpleFunction ('mytest', function () {
           return "This is a test.";
       });
       $this->grav['twig']->twig->addFunction($function);                   
   }
// ...
And use it in Twig with {{ mytest() }}.
              
              
              1 Like
            
            
          @punchfighter in addition to @Utis comment read this (extend twig)