How to Pass a PHP Interview: Advanced Questions and Answers

How to Pass a PHP Interview: Top Advanced-Level Questions

An interview for a Senior PHP Developer position is not just a syntax check. The employer evaluates architectural thinking, understanding of the language's internals, experience with high-load systems, and knowledge of modern standards. In this article, we will break down key advanced-level questions with practical examples that will help you confidently pass a technical interview.

1. SOLID in PHP: Principles and Examples of Violations

The question about SOLID is a classic in any interview. It's important not just to list the principles, but also to show how their violation leads to problems and how to fix them.

Single Responsibility Principle (SRP). A class should have only one reason to change. Consider a typical violation:

class User {    public function save() { /* saving to DB */ }    public function sendEmail() { /* sending an email */ }    public function generateReport() { /* generating a report */ }}

Here, the User class is responsible for saving, notifications, and reports. The correct approach is to move each responsibility into a separate service:

class UserRepository {    public function save(User $user) { /* ... */ }}

class MailService {    public function sendWelcomeEmail(User $user) { /* ... */ }}

class ReportGenerator {    public function generate(User $user) { /* ... */ }}

Open/Closed Principle (OCP). Classes should be open for extension but closed for modification. Instead of if-else, use polymorphism:

// OCP violationclass PaymentProcessor {    public function pay(string $type) {        if ($type === 'credit_card') { /* ... */ }        elseif ($type === 'paypal') { /* ... */ }    }}

// Correct solutioninterface PaymentMethod {    public function process();}

class CreditCardPayment implements PaymentMethod {    public function process() { /* ... */ }}

class PayPalPayment implements PaymentMethod {    public function process() { /* ... */ }}

class PaymentProcessor {    public function pay(PaymentMethod $method) {        $method->process();    }}

In an interview, be sure to mention that for PHP, the Dependency Inversion Principle (DIP) is also important — depending on abstractions, not concrete classes. Show an example with Dependency Injection:

class OrderService {    private PaymentMethod $paymentMethod;        public function __construct(PaymentMethod $paymentMethod) {        $this->paymentMethod = $paymentMethod;    }        public function checkout() {        $this->paymentMethod->process();    }}

2. Design Patterns: When and Why

Questions about patterns in an interview test whether you can apply standard solutions to real-world problems. It's not enough to just name the pattern — you need to explain what problem it solves.

Singleton. Used for objects that should exist in a single instance (e.g., a database connection). But be prepared for criticism: many consider Singleton an anti-pattern due to hidden dependencies and testing issues.

class Database {    private static ?Database $instance = null;        private function __construct() { }        public static function getInstance(): Database {        if (self::$instance === null) {            self::$instance = new self();        }        return self::$instance;    }}

Factory Method. Creates objects without specifying the exact class. It is especially useful when working with different types of connections:

interface Connection { }class MysqlConnection implements Conn