如何修改/扩展全局TWIG变量？

I know you can set global variables in TWIG this way :

# config.yml
# Twig Configuration
twig:
    debug:            "%kernel.debug%"
    strict_variables: "%kernel.debug%"
    cache : false
    globals :
        server : 127.0.0.1

Now what I want is to create a custom variable server that uses the preexistent global variable app.request.host. I can reference that variable in a TWIG template, but how can I do that in a YML file ?

In one sentence : I want to create a global TWIG variable from another one.

Why don't you go with a parameter. Just put it in you parameters.yml

server: 127.0.0.1

after that you can refer to it in yml files like this:

"%server%"

add it to globals like this:

twig:
    globals:
        server: '%server%'

if you need it in controller, then use this approach:

$this->getParameter("server")