Use values from fields in shortcode

Hi Tijmen, thanks for your hard work & patiente with us.

I want to use the value of a custom_meta_field in other shortcode. Im doing this in functions.php

$var1 = "<%= some_value_different_per_store%>;
echo do_shortcode('[groups_join group="' .$var1. '"]');

([groups_join group=”‘ X ‘”] show a button for join the group of the store)

But, <%= some_value %> isn’t a real value, because is interpreted in the client, Right?
How i can get this value at server?

Sorry for my english, regards and thanks again.

https://www.ads-software.com/plugins/wp-store-locator/