Fix usage of static variables

[jamieburchell]

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Fixes problematic use of static variables in inherited methods in PHP >= 8.1

See:

• https://www.php.net/manual/en/migration81.incompatible.php
• https://github.com/bcit-ci/CodeIgniter/pull/6173#issuecomment-2113501165
• https://github.com/codeigniter4/CodeIgniter4/pull/5262/files

Notes:

• There is a mixture of "char" and "chr" naming conventions so I just chose one.
• There are other instances where static variables are used in database classes but I do not know if these are problematic. I think not if they are introduced in what are essentially final classes, but I have not tested or verified.