lib/Twig/lib/Twig/Environment.php
public function addFunction($name, $function = null)
    {
        if (!$name instanceof Twig_SimpleFunction && !($function instanceof Twig_SimpleFunction || $function instanceof Twig_FunctionInterface)) {
            throw new LogicException('A function must be an instance of Twig_FunctionInterface or Twig_SimpleFunction.');
        }

        if ($name instanceof Twig_SimpleFunction) {
            $function = $name;
            $name = $function->getName();
        } else {
            @trigger_error(sprintf('Passing a name as a first argument to the %s method is deprecated since version 1.21. Pass an instance of "Twig_SimpleFunction" instead when defining function "%s".', __METHOD__, $name), E_USER_DEPRECATED);
        }

        if ($this->extensionInitialized) {
            throw new LogicException(sprintf('Unable to add function "%s" as extensions have already been initialized.', $name));
        }

        $this->staging->addFunction($name, $function);
    }
public function addFunction($name, $function = null)
    {
        if (!$name instanceof Twig_SimpleFunction && !($function instanceof Twig_SimpleFunction || $function instanceof Twig_FunctionInterface)) {
            throw new LogicException('A function must be an instance of Twig_FunctionInterface or Twig_SimpleFunction.');
        }

        if ($name instanceof Twig_SimpleFunction) {
            $function = $name;
            $name = $function->getName();
        } else {
            @trigger_error(sprintf('Passing a name as a first argument to the %s method is deprecated since version 1.21. Pass an instance of "Twig_SimpleFunction" instead when defining function "%s".', __METHOD__, $name), E_USER_DEPRECATED);
        }

        if ($this->extensionInitialized) {
            throw new LogicException(sprintf('Unable to add function "%s" as extensions have already been initialized.', $name));
        }

        $this->staging->addFunction($name, $function);
    }
Registers a Function.

@param string|Twig_SimpleFunction                 $name     The function name or a Twig_SimpleFunction instance
@param Twig_FunctionInterface|Twig_SimpleFunction $function