/**
 * Class Cfg
 *
 * Handles configuration management, database connection, and user access control for the application.
 * 
 * Responsibilities:
 * - Defines constants for user identifiers, Active Directory (AD) groups, and access levels.
 * - Manages valid AD groups and their corresponding access levels.
 * - Provides localized week day names and acronyms.
 * - Establishes a PDO connection to the database and retrieves configuration values.
 * - Handles OAuth2 configuration and environment variables.
 * - Determines user access levels and permissions based on AD group membership.
 * - Provides utility methods for session simulation and maintenance mode.
 *
 * Properties:
 * @property array $validADGroups      Mapping of AD group names to access levels.
 * @property array $values             Configuration values loaded from the database.
 * @property array $oauth2             OAuth2 configuration values.
 * @property \PDO|object $pdo          PDO database connection or empty object on failure.
 * @property object $servidor          Environment configuration object.
 * @property static array $weekDays    Localized names of the days of the week.
 * @property static array $acronymsOfDays Acronyms for the days of the week.
 *
 * Methods:
 * - __construct(): Initializes environment, database connection, and loads configuration.
 * - getConfigValuesFromDB(): Loads configuration values from the database.
 * - isOauthKey(string $nameInDB): Checks if a config key is an OAuth key.
 * - extractOauthKey(string $nameInDB): Extracts the OAuth key name.
 * - oauthValues(array $row, string $key, array &$oauth): Processes OAuth-related config values.
 * - oauthEnv(): Returns OAuth2 environment configuration.
 * - value(string $name): Retrieves a configuration value.
 * - DSN(string $cs = 'UTF8'): Builds the DSN string for PDO.
 * - user(): Returns the database username.
 * - pswd(): Returns the database password.
 * - isValidADGrup(): Checks if the current user belongs to a valid AD group.
 * - canAccessTo(string $grup): Checks if the user can access a specific group.
 * - getUserLevel(): Determines the user's access level based on AD group membership.
 * - getLevelGrups(int $level): Retrieves accessible groups for a given access level.
 * - groupInAccesses(string $group, mixed $accesses): Checks if a group is in the access list.
 * - isADGroupInUserGroups(mixed $adGroupToSearch, mixed $userGroups): Checks if an AD group is in the user's groups.
 * - static fakeSession(string $oauth2TokenSessionVar): Simulates a user session for testing.
 * - static noWeb(string $msg): Returns a maintenance page with a custom message.
 */