Fix phpstan warnings

Hi !

Fixed all phpstan reported issues.

MR ready for review.

Hi @nfabing
thanks for your work. but it creates new phpcs issues. e.g. Class property $splashify_injection should use lowerCamel naming without underscores

Hi @astonvictor

Thanks for the review, I fixed all newly reported phpcs issues.

thanks

Automatically closed - issue fixed for 2 weeks with no activity.