If you want to use it in QueryBuilder you must :
1) add DQL functions GroupConcat: GroupConcat
2 ) Registering GroupConcat :DQL User Defined Functions
another alternative is to use NativeQuery :Native SQL
In symfony2 registering a function DQL it’s very simple just add GROUP_CONCAT in config.yml like:
entity_managers:
default:
dql:
string_functions:
GROUP_CONCAT: YourBundle\Query\Mysql\GroupConcat
For more information visit Registering Custom DQL Functions