Review the `AbstractVariableRestrictionsSniff` class

Review the `AbstractVariableRestrictionsSniff` class for the following in as far as relevant:

- [ ] Code style independent sniffing / Correct handling of quirky code
Typical things to add tests for and verify correct handling of:
- [ ] Nested function/closure declarations
- [ ] Nested class declarations
- [ ] Comments in unexpected places
- [ ] Variables being assigned to via `list` statements
- [ ] Multiline text strings
- [ ] Text strings provided via heredoc/nowdoc
- [ ] Use of short open tags
- [ ] Using PHP close tag as end of statement
- [ ] Inline control structures (without braces)
- [ ] Code simplifications which can be made using PHPCSUtils
- [ ] Sniff stability improvements which can be made using PHPCSUtils
- [ ] Correct handling of modern PHP code
Typical things to add tests for and verify correct handling of (where applicable):
- [ ] PHP 5.0 Try/catch/finally (PHP 5.5) and exceptions
- [ ] PHP 5.3 Namespaced code vs code in the global namespace
- [ ] PHP 5.3 Use import statements, incl aliasing
- [ ] PHP 5.3 Short ternaries
- [ ] PHP 5.3 Closures, incl closure use
- [ ] PHP 5.4 Short arrays
- [ ] PHP 5.5 Class name resolution using `::class`
- [ ] PHP 5.5 List in foreach
- [ ] PHP 5.5/7.0 Generators using yield and yield from
- [ ] PHP 5.6 Constant scalar expressions
- [ ] PHP 5.6 Importing via `use function/const`
- [ ] PHP 7.0 Null coalesce
- [ ] PHP 7.0 Anonymous classes
- [ ] PHP 7.0 Scalar and return type declarations
- [ ] PHP 7.0 Group use statements
- [ ] PHP 7.1 Short lists
- [ ] PHP 7.1 Keyed lists
- [ ] PHP 7.1 Multi-catch
- [ ] PHP 7.1 Nullable types
- [ ] PHP 7.3 List reference assignments
- [ ] PHP 7.4 arrow functions
- [ ] PHP 7.4 numeric literals with underscores
- [ ] PHP 7.4 null coalesce equals
- [ ] PHP 7.4 Typed properties
- [ ] Various versions: trailing comma's in function calls, group use, function declarations, closure use etc

Other:
- [ ] Review violation error vs warning
- [ ] Review violation severity
- [ ] Review violation message, consider adding a link
- [ ] Check open issues related to the sniff
- [ ] Review PHPDoc comments

Sniff basics, but changes need to be lined up for next major release:
- [ ] Inappropriate use of `public` properties (#234)
- [ ] Modular error codes (unique error code for each distinct message)

Once PHPCS/PHPCSUtils supports this:
- [ ] PHP 8.0 Constructor property promotion
- [ ] PHP 8.0 Union types
- [ ] PHP 8.0 `match` expressions
- [ ] PHP 8.0 Nullsafe operator
- [ ] PHP 8.0 Named arguments
- [ ] PHP 8.0 Single token namespaced names

Bắt đầu bằng cách tìm lớp AbstractVariableRestrictionsSniff và mọi bài kiểm tra liên quan. Thu hẹp việc xem xét vào một mục cụ thể trong checklist, sau đó xác minh hành vi và các bài kiểm tra liên quan; để được coi là hoàn tất, cần có một tập hợp thay đổi đã thống nhất và có phạm vi cụ thể, thay vì hoàn thành toàn bộ việc xem xét.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.