Phpstan hasn't identified any errors in Samples in a long time. But, as it turns out, one particular error is suppressed:
```
Variable $helper might not be defined.
```
This error would be generated by almost all samples, and the errors that it suppresses will become problematic if we move to Level 10. We are not yet committed to doing that, but it is pretty easy to write a script to change the samples so that error no longer happens, and there isn't really any reason to delay doing so. The results of that script constitute this PR.
Fix#4411. User copied some code from a PHPExcel program which set DisplayAsBlanks to `0`. This resulted in what Excel deemed a corrupt spreadsheet using PhpSpreadsheet. The only values allowed for that field are `gap`, `zero` (not `0`), and `span`. PHPExcel used `0` as a default, and got away with it because it ignored the value entirely when writing out the spreadsheet, using `gap` all the time.
I had to choose between throwing an exception and just using the default when an attempt is made to set that property to an invalid value. An exception just seems more punitive than helpful to me, especially if we want people to migrate from PHPExcel, which still seems to have a large user base. So I've gone with using `gap` in place of an invalid value. Note that, according to https://learn.microsoft.com/ru-ru/openspecs/office_standards/ms-oe376/b5c5c694-21d9-437c-9a4a-21e0e843eed8, `gap` is used as the default whenever it is permitted for the chart in question; and, when it isn't permitted, the chart will use its default method (which will always be `zero`).
There were no tests nor samples for this property. All the tests and samples which use it use only `gap`. I have added a small test, and a new sample to illustrate the difference between the 3 options.
The samples have become unwieldy when running them from a browser. In particular, the drop-down lists are fixed size with no scrolling, and many of them are now just too large. I have moved all the Calculation samples up a level, and broken several categories (Basic, Chart, DateTime, Engineering, Financial, and Reader) into several pieces.
Convert-Online (now found in the Engineering category) had a number of different problems which are now resolved. It is the only member with any significant code change.
oleibman