Use strict typing in all PHP files (declare(strict_types=1);)

Declaring all PHP files as strict typed will prevent PHP from coercing types, thus potentially catching errors and implicit assumptions early.

This will require adding declare(strict_types=1); right after the opening <?php like so:

<?php

declare(strict_types=1);

...

Once that's done, it may surface some errors, which might require some (hopefully) minimal work to fix.

This should be done before #3305735: Type hint method/function parameters, return types, and class properties.