Closes #6541 Add PHPStan CI & level 0 errors

[Tabrisrp]

Description

• Adds PHPStan to our CI
• Fixes level 0 errors reported by PHPStan

Fixes #6541

Type of change

• Enhancement (non-breaking change which improves an existing functionality).

Checklists

Code style

• I wrote self-explanatory code about what it does.
• I wrote comments to explain why it does it.
• I named variables and functions explicitely.
• I did not introduce unecessary complexity.

Observability

• I handled errors when needed.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.01% (target: -0.10%)	❌ 30.00% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (cccbadf)	37177	14376	38.67%
Head commit (1da63ca)	37192 (+15)	14377 (+1)	38.66% (-0.01%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6617)	10	3	30.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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