Declare strict_types in all PHP files

Problem/Motivation

From the PHP manual:

By default, PHP will coerce values of the wrong type into the expected scalar type declaration if possible. For example, a function that is given an int for a parameter that expects a string will get a variable of type string.

This silent type-casting is often unintentional and can result in bugs.

If we set declare(strict_types=1); at the top of a file, then "only a value corresponding exactly to the type declaration will be accepted, otherwise a TypeError will be thrown. The only exception to this rule is that an int value will pass a float type declaration."

Proposed resolution

Set declare(strict_types=1); in the module files that we don't get do that. Review variable usage in those files before committing. Make sure tests still pass afterwards.

As of commit c3f1e98 from 2026-05-06, there are 21 PHP files in this module that we do not declare strict types in (using ripgrep, rg -g '*.php' --files-without-match -i -F -e 'declare(strict_types=1);' will find the list of files.

Remaining tasks

1. Merge request - merge request !16 created by @mparker17 in #2
2. Community/Maintainer review - skipped by @mparker17 in #3
3. Commit - merged by @mparker17 in #4
4. Release

User interface changes

None.

API changes

None.

Data model changes

None.