NaN and Infinity are a bit tricky

For PHP these numbers are identified by the `PHP_INT_MIN` usually, but Erlang hasn't a limit for integers so, I did the implementation as:

```erlang
to_int(infinity) -> -9223372036854775808;
to_int(nan) -> -9223372036854775808;
```

Maybe it should be fixed in another way (see `ephp_data.erl` for further details).