public function getKeys($env, $prefix = null)
	{
		$this->validateEnvironment($env);

		$env  = $env === self::APP ? self::EXT : self::APP;
		$keys = array_keys($this->definitions[$env]);

		if ($prefix !== null) {
			$keys = array_map(function ($key) use ($prefix) {
				return $prefix . $key;
			}, $keys);
		}

		return $keys;
	}