Add PHPStan

[swissspidy]

Working towards higher levels

[codecov]

Codecov Report

Attention: Patch coverage is 64.86486% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 94.07%. Comparing base (12e97ae) to head (eeeee28).
Report is 6 commits behind head on main.

Files	Patch %	Lines
inc/functions.php	64.86%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #961      +/-   ##
==========================================
- Coverage   95.42%   94.07%   -1.36%     
==========================================
  Files           5        5              
  Lines         612      641      +29     
  Branches       39       39              
==========================================
+ Hits          584      603      +19     
- Misses         28       38      +10     

Flag	Coverage Δ
default	91.79% <64.86%> (-1.76%)	⬇️
js	100.00% <ø> (ø)
multisite	91.79% <64.86%> (-1.76%)	⬇️

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[szepeviktor]

So sad to see low level code (HTTP request processing) mixed with high level code (language selection).