Fix #78296: is_file fails to detect file

If we're constructing extended-length paths (i.e. paths prefixed with
`\\?\`), we have to replace all forward slashes with backward slashes,

Fix #78296: is_file fails to detect file

If we're constructing extended-length paths (i.e. paths prefixed with
`\\?\`), we have to replace all forward slashes with backward slashes,
because the former are not supported by Windows for extended-length
paths.

The more efficient and likely cleaner alternative solution would be to
cater to this in `php_win32_ioutil_normalize_path_w()` by always
replacing forward slashes, but that might break existing code. It
might be sensible to change that for `master`, though.

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