PHP is a server-side scripting language designed to create dynamic, data-driven web applications. When a browser requests a .php page, the server executes its code, builds HTML (or JSON for APIs), and sends the result back to the client. Unlike JavaScript, which runs inside the browser, PHP runs entirely on the server. This separation gives PHP control over sessions, authentication, forms, business logic, and database interactions. At its heart, PHP is a request–response engine: receive an HTTP request, process it, generate output, terminate. For beginners, PHP is approachable because its syntax resembles English instructions. A simple echo "Hello"; line prints text, and variables, arrays, loops, and functions feel familiar to anyone with basic programming exposure. The language also includes hundreds of built-in functions for string operations, file handling, date/time processing, and common tasks. Most people first encounter PHP while adding login systems, managing forms, or building dynamic pages using MySQL. From a backend developer’s perspective, PHP becomes more than a scripting language; it becomes a toolset for building structured, secure, scalable applications. Developers use PDO with prepared statements to prevent SQL injection; they maintain sessions securely; they manage cookies; handle file uploads; and integrate with databases such as MySQL, PostgreSQL, and Redis. PHP-FPM (FastCGI Process Manager) powers execution in production environments, offering worker pools, configurable process handling, and optimized performance. With frameworks like Laravel, Symfony, and Yii, the language evolves into a full web platform, complete with routing, middleware, ORM layers, queues, caching, API development, and event systems. Composer, PHP’s dependency manager, handles libraries and autoloading using standards like PSR-4. Modern PHP also embraces object-oriented programming with classes, interfaces, traits, namespaces, and exceptions. From a senior architect’s viewpoint, PHP fits neatly into modern web ecosystems, even at scale. It works well behind Nginx or Apache, scales horizontally across multiple servers, and benefits from stateless request handling, which means each request is isolated and memory leaks are short lived. Redis and Memcached strengthen caching and session storage. Message queues like Kafka, RabbitMQ, or AWS SQS allow PHP services to offload long tasks. With OPcache enabling bytecode caching, PHP 7 and 8 deliver excellent performance, often matching or surpassing other backend languages in real-world workloads. PHP can power monoliths, modular monoliths, microservices, or serverless functions (using Bref on AWS Lambda). Deployments use Docker, Kubernetes, ECS, and CI/CD pipelines. PHP also integrates easily with API gateways, reverse proxies, and load balancers. Its weaknesses mainly involve limited native multithreading and partial async capability, though projects like Swoole and ReactPHP push the language toward event-driven performance patterns. For interviews, PHP knowledge spans fundamentals to architectural thinking. A candidate must know how requests are handled, how sessions and cookies work, how to secure forms with CSRF tokens, how to prevent SQL injection, when to use abstract classes vs interfaces, how Composer manages autoloading, what MVC means in PHP frameworks, how PHP-FPM manages process pools, why OPcache drastically improves performance, how to structure controllers/services/repositories, how to optimize queries and database indexes, how to store passwords using password_hash(), how to handle file uploads securely, and how to write scalable APIs. Senior interviews explore design patterns (Repository, Factory, Dependency Injection), clean architecture, SOLID principles, API rate-limiting, caching strategies, and using queues for long-running tasks. As a single, continuous explanation: PHP is both a beginner-friendly entry point into web development and a mature, production-grade backend technology used across millions of websites. It evolves continuously, maintains a vast ecosystem, and integrates seamlessly with modern cloud infrastructures. Its core strength lies in its simplicity for small tasks and its robustness for large applications.

Using tools like Memcached or Redis. Memcached is used for simple key-value caching with LRU eviction. Redis is used for more advanced data structures and persistence.

Multi-layer caching: Local cache (variables/structs), Distributed cache (Redis/Memcached). Cache-aside pattern is common.