The goal set by ContainerInterface is to standardize how frameworks and libraries make use of a container to obtain objects and parameters. An entry identifier is any PHP-legal string of at least one character that uniquely identifies an item within a container. An entry identifier is an opaque string, so callers should not assume that the structure of the string carries any semantic meaning. The Psr\Container\ContainerInterface exposes two methods, get and has. get takes one mandatory parameter: an entry identifier, which must be a string. get can return anything (a mixed value), or throw a NotFoundExceptionInterface if the identifier is not known to the container. Two successive calls to get with the same identifier should return the same value. has takes one unique parameter: an entry identifier, which must be a string. has must return true if an entry identifier is known to the container and false if it is not.