public static function appPath(?string $app = null): ?string
    {
        self::initialize();
        if ($app === null) {
            $app = '*';
        }
        if (isset(self::$repositories[$app])) {
            return call_user_func([self::$repositories[$app], 'getPath']);
        }
        return null;
    }
public static function appPath(?string $app = null): ?string
    {
        self::initialize();
        if ($app === null) {
            $app = '*';
        }
        if (isset(self::$repositories[$app])) {
            return call_user_func([self::$repositories[$app], 'getPath']);
        }
        return null;
    }
public static function appPath ( ? string $ app = null ) : ? string { self :: initialize ( ) ; if ( $ app === null ) { $ app = '*' ; } if ( isset ( self :: $ repositories [ $ app ] ) ) { return call_user_func ( [ self :: $ repositories [ $ app ] , 'getPath' ] ) ; } return null ; }
App path.

@param string $app Default null which means default app '*'.
@return string
App path .