PHP nullable typehinting not supported now

WTF: 
https://wiki.php.net/rfc/nullable_types

Error:
```
File: /app/vendor/symfony/config/Definition/BaseNode.php
> [Error] Syntax error, unexpected '?', expecting T_VARIABLE on line 39. Processing aborted.
```

Code:
```
    public function __construct(?string $name, NodeInterface $parent = null, array $values = array())
```

