SplFileObject: key() returns wrong value

### Description

The following code: https://3v4l.org/RHFfI#v8.1.4

Resulted in this output:
```
0: line 0
0: line 1
1: line 2
2: line 3
3: line 4
```

But I expected this output instead:
```
0: line 0
1: line 1
2: line 2
3: line 3
4: line 4
```


### PHP Version

PHP 8.1.4

### Operating System

Windows 11