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.
Jpgraph does not use namespaces. Neither do the Samples members of this project. As a result, Scrutinizer complains about 13 cases of `use PhpOffice\PhpSpreadsheet\Chart\Legend;`, since this theoretically could cause the use of two different Legend classes in the empty namespace. It suggests using a class alias for Legend, which this PR does in every applicable case. It is a regrettable problem, but it is easy to fix.
This introduce a helper class that should be used to log things,
avoiding a lot of boilerplate code.
Also all output are made in /tmp folder instead of beside the script
itself. This is because there is a high chance that the folder containing
the script is not writtable by webserver. So using the /tmp folder
makes it more likely to works in a variety of setup.
oleibman
MarkBaker
Adrien Crivelli
Zharikov Viktor