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.
Custom properties would be better in this context using j (no leading 0) rather than d (leading 0) for day of month.
Properties has the same problem, and also needs backslashes before p to make it part of a <sup> tag.
A continuation of PR #3859. Change code that would be flagged if we were to run Phpstan at level 9 (we currently run level 8). I may or may not follow up with source code (454 level-9 problems remain for src), but there is no reason to avoid the effort for samples.
No changes are made to src.
oleibman
Adrien Crivelli