Fix#4537. PhpSpreadsheet currently changes apostrophes in text values to `'`. This is perfectly valid Xml. Issue was opened because R does not handle this correctly; this is unquestionably a bug on R's part. So I was not inclined to do anything about it. However ...
User suggested a change to how `htmlspecialchars` was called. Investigating the use of that routine in PhpSpreadsheet, I found that there was some double escaping going on for cells whose type was set to `TYPE_INLINE` - `htmlspecialchars` escaped the string correctly, but it was later written as Xml using a method which escaped the data a second time. So, a real bug in PhpSpreadsheet after all.
There was one call to `htmlspecialchars` in `Shared\XmlWriter`. I replaced `writeRaw(htmlspecialchars(...))` with `text(...)`. And one call in `Writer\Xlsx\Worksheet`, the source of the double escaping bug above; the call to `htmlspecialchars` can just be eliminated there.
Making those changes, the only remaining calls to `htmlspecialchars` are in `Writer\Html`, where they belong. As a bonus, apostrophes now wind up unescaped, so R will be satisfied (even though they should fix their bug).
oleibman