How to concatenate a parameter value with a constant string in Yaml

You don't need a concatenation character, you can append the constant directly after the parameter. For example, for a service definition in services.yml:

my.service:
    class: "%my.service.class%"
    arguments:     
      - "@logger", 
      - "%some_parameter%", 
      - "%parameter_url%?wsdl", 

If you are looking to concatenate two parameters. You can do it this way:

parameters.yml

parameter_url: XXXXX parameter_type: ?wsdl

services.yml

my.service:
    class: %my.service.class%
    arguments:     
        - @logger, 
        - %some_parameter%, 
        - '%parameter_url%%parameter_type%',