PHP 8.3 support

[Antnee]

Now tests against:

• PHP 8.0
• PHP 8.1
• PHP 8.2
• PHP 8.3

Have also:

• Moved the functions to their own test file for better separation
• Corrected the incorrect function signature on filter()
• Updated check actions to latest versions

[coveralls]

Pull Request Test Coverage Report for Build 7680270683

• 0 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.6%) to 100.0%

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Totals
Change from base Build 4541241434:	1.6%
Covered Lines:	122
Relevant Lines:	122

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💛 - Coveralls

[codeclimate]

Code Climate has analyzed commit 5e51b52 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (100% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (1.2% change).

View more on Code Climate.