http://blog.arturu.it/libro/sviluppo-php-concerto-un-framework-didattico/67/abstract-singleton
http://blog.arturu.it/libro/sviluppo-php-concerto-un-framework-didattico/67/abstract-singleton
http://blog.arturu.it/libro/sviluppo-php-concerto-un-framework-didattico/67/abstract-singleton