This guide explains how to debug and log errors in PHP using logging functions and Sentry. It covers the basics of PHP exceptions and errors, including fatal, warning, and notice levels, as well as how to use try-catch blocks to handle errors. The guide also discusses the importance of testing, logging, debugging, and monitoring in detecting and preventing errors. Additionally, it provides an overview of how to log messages to the browser and terminal for simple debugging, and how to set up an error monitoring dashboard and alerting system with Sentry.