What new features did PHP 7 and PHP 8 introduce that changed how you write code?

The language changed substantially, and interviewers use this to date your experience.

PHP 7 brought:

• Scalar type declarations and return types, plus declare(strict_types=1) to make them enforced rather than coerced.
• The null coalescing operator ??, replacing the isset() ? : dance.
• The spaceship operator <=> for comparison functions.
• Errors as exceptions, so fatal errors became catchable.
• Roughly double the performance of PHP 5.6, which is why the upgrade was worth doing on its own.

PHP 8 brought:

• Constructor property promotion — declaring and assigning properties in the signature, which removes a great deal of boilerplate.
• Named arguments, so createUser(admin: true) is readable at the call site.
• Union types (int|string), and readonly properties in 8.1.
• Enums in 8.1 — real type-safe enumerations instead of class constants.
• Match expressions, which return a value and use strict comparison with no fall-through.
• The nullsafe operator ?->, and attributes for structured metadata instead of docblock parsing.
• JIT compilation, which helps CPU-bound work more than typical web requests.

Note: The most useful thing to say is which of these changed your habits. Promoted constructors, enums, and readonly properties together make immutable value objects genuinely pleasant, which was awkward before.