SplFileInfo#getBasename() truncates first character

### Description

The following code:

```php
<?php
$file = new \SplFileInfo('foo');
var_dump($file->getBasename());
```

Resulted in this output:
```
string(2) "oo"
```

But I expected this output instead:
```
string(3) "foo"
```

See https://3v4l.org/VmBqd/rfc#vgit.master.

### PHP Version

current master

### Operating System

_No response_