app/config/config.yml

config.yml

framework:
    #...
    session:
        default_locale: %locale%
        lifetime:       3600
        auto_start:     true
        storage_id:     session.storage.pdo
    #...

parameters:
    pdo.options: []
    pdo.db_options:
        db_table: session

services:
    session.storage.pdo:
        class:    Symfony\Component\HttpFoundation\SessionStorage\PdoSessionStorage
        arguments:
            - @pdo
            - %pdo.options%
            - %pdo.db_options%

    pdo:
        class:    PDO
        arguments:
            dsn:      "mysql:dbname=%database_name%"
            user:     %database_user%
            password: %database_password%