If you're using Claude or another AI assistant for PHP development without a CLAUDE.md , you've seen this pattern: the AI generates working code, but it's PHP 5-era style — no type hints, mysql_* functions, procedural globals, or die() error handling.

PHP has evolved dramatically. PHP 8.x has union types, enums, fibers, named arguments, match expressions, and readonly properties. Modern PHP is a different language from what most AI training data contains. Without explicit rules, AI defaults to the lowest common denominator.

A CLAUDE.md file at your repo root tells the AI which decade you're working in. Here are the 13 rules that have the highest impact.

Rule 1: Enforce strict types on every file

Every PHP file must start with `declare(strict_types=1);` immediately after the opening tag. No exceptions — including scripts, controllers, helpers, and test files.

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Without strict_types , PHP silently coerces values.…