Key PHP Interview Topics: HTTP Status Codes, Pass‑by‑Value/Reference, Design Patterns, XSS, CGI/FastCGI/PHP‑FPM, MVC, Garbage Collection, CLI Lifecycle, Memory Management, Arrays, Dependency Injection, OOP

1. HTTP status codes : 302 temporary redirect, 401 unauthorized, 403 forbidden, 500 internal server error.

2. Pass‑by‑value vs. pass‑by‑reference : value copies the argument, reference passes the variable’s address, so modifications affect the original.

3. Design patterns :

Creational: Prototype, Factory, Singleton, Builder, Abstract Factory.

Structural: Adapter, Bridge, Flyweight, Facade, Proxy, Composite, Decorator.

Behavioral: Memento, Chain of Responsibility, Command, Iterator, Mediator, State, Template Method, Visitor, Observer, Strategy.

4. Code management : Teams commit code to a version‑control server; a project lead manages versions to enable rollback and improve development efficiency.

5. XSS attacks : Attackers inject malicious scripts into web pages; mitigation is done by applying htmlspecialchars() to user input.

6. CGI, FastCGI, PHP‑FPM relationship :

Web Server (e.g., Apache, Nginx) serves static files directly.

For a PHP request, the server launches a CGI program (the PHP interpreter) which reads php.ini, processes the script, and returns the result via the CGI protocol.

Definitions: Web Server, Web Application, CGI, FastCGI, PHP‑CGI, PHP‑FPM.

7. MVC architecture : Model handles data operations, View renders the UI, Controller contains business logic.

8. PHP garbage collection : Uses reference counting; an object is freed when its reference count drops to zero.

9. CLI lifecycle stages :

Stage

Function

Purpose

Module initialization

php_module_startup()

Initialize PHP core and Zend engine.

Request initialization

php_request_startup()

Prepare request data for FPM workers.

Script execution

php_execute_script()

Parse PHP code and generate an abstract syntax tree.

Request shutdown

php_request_shutdown()

Execute cleanup at the end of a request.

Module shutdown

php_module_shutdown()

Run when the process terminates.

10. PHP‑FPM operation : FastCGI is the protocol between web server and PHP; PHP‑FPM implements FastCGI with a master‑worker model. Process management modes include static (fixed workers), dynamic (adjusts workers based on load), and ondemand (spawns workers on demand). The master runs an event loop handling I/O and timers.

11. Memory allocation : PHP pre‑allocates a memory pool to reduce OS‑level allocation overhead for small requests.

12. PHP array implementation : Implemented as a hash table (hash map) providing average O(1) lookup time.

13. Dependency injection : Services receive their dependencies from the outside (often via reflection), reducing coupling and allowing code to depend on interfaces rather than concrete implementations.

14. Object‑oriented programming : Emphasizes encapsulation, inheritance, polymorphism, and follows SOLID principles (Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion).

Related article: “PHP interview questions – application problems collection”.