Explore a comprehensive case study on upgrading a large-scale PHP application to PHP7 at Wayfair, one of the world's largest online home goods retailers. Dive into the intricate testing strategies and tools employed to ensure a smooth transition for a 3.5 million lines of code application serving 2 million daily visitors. Learn about the limitations of unit tests and discover a range of testing methodologies, including static analysis, automated testing, and system testing. Gain insights into planning tools, risk assessment, and the use of various testing tools such as PHPUnit, GDB, and ValGrind. Understand the importance of consistency, memory management, and continuous testing in maintaining a complex PHP application. Benefit from practical examples and lessons learned during this successful upgrade project, and discover how to apply these testing strategies to your own PHP applications.