More PHPStan level 6 cleanups, some *.php files

[bpfoley]

This is mostly adding function type hints, but in a few cases we:

• Change the function signature/behaviour to make a little more sense
• Add a variation of the function to handle polymorphic functions that can work on strings or associative arrays
• Restrict the function return value (eg "" instead of null) to match the intended behaivour
• Remove a little unused code
• Fix a couple of erroneous function return types or array shape definitions (my bad)
• Use a DTO instead of an array where it seems worth it
• Express a variadic function more directly rather than using func_get_args