PHP 4? OMG! A small vademecum for obsolete software migration.

This document provides guidance for migrating from PHP 4 to PHP 5. It recommends creating a comprehensive test suite to test core functionality, workflows, third party interactions, and classes. It also suggests checking for outdated libraries, code smells, dependencies, and PHP settings. The migration process should happen iteratively, upgrading PHP and the system in stages while using the test suite and fixing any issues. Once the core upgrade is complete, more refactoring is needed such as improving tests, following standard patterns, using frameworks, removing unused code, and more. The overall process helps ensure a smooth transition from obsolete to current software.