Wrong return value documented for substr()

From manual page: https://php.net/function.substr

---

In example #2 under the `length` parameter the following line is stated:

```php
$rest = substr("abcdef", 4, -4);  // gibt false zurück
```

This is wrong as the correct return value now is `""`.

The correct line should be:
```php
$rest = substr("abcdef", 4, -4);  // gibt "" zurück; vor PHP 8.0.0 wurde false zurückgegeben
```