10 Must Ask PHP Interview Questions For Hiring Managers

Posted on April 3 2024 by Interview Zen Team

Introduction

PHP, an acronym for Hypertext Preprocessor, stands as a foundational pillar in web development. Originally designed to create dynamic web pages, PHP has evolved into a robust server-side scripting language that powers some of the world’s most popular websites, including Facebook, Wikipedia, and WordPress.

According to W3Techs, PHP is used by 76.8% of all websites with a known server-side programming language, making it the most widely used server-side language on the web. This dominance makes PHP expertise highly valuable in the job market.

This comprehensive guide provides hiring managers with essential PHP interview questions designed to evaluate candidates’ understanding of core concepts, best practices, and practical application skills in modern web development.

What is PHP?

PHP is a popular general-purpose scripting language that is especially suited to web development. It’s fast, flexible, and pragmatic, powering everything from blogs to the most popular websites in the world. PHP code is executed on the server, generating HTML which is then sent to the client.

Key characteristics of PHP include:

  • Open source: Free to use and widely supported
  • Server-side: Code executes on the web server
  • Cross-platform: Runs on various operating systems
  • Database friendly: Excellent integration with databases like MySQL, PostgreSQL
  • Large ecosystem: Extensive libraries and frameworks available

Top 10 Must-Ask PHP Interview Questions

1. What is the difference between include, require, include_once, and require_once?

This question tests understanding of PHP’s file inclusion mechanisms.

Example Answer: “All four are used to include files:

  • include: Includes file, continues execution if file not found (warning)
  • require: Includes file, stops execution if file not found (fatal error)
  • include_once/require_once: Same as above but includes file only once Use require_once for critical files like configuration, include for optional templates.”

2. Explain the difference between == and === operators.

Understanding PHP’s type comparison is crucial for avoiding bugs.

Example Answer: “== performs type juggling and compares values after type conversion. === compares both value and type without conversion.

'5' == 5   // true (string converted to int)
'5' === 5  // false (different types)
0 == false // true
0 === false // false

Always prefer === for precise comparisons.”

3. What are PHP superglobals? Name and explain them.

Superglobals are built-in variables available in all scopes.

Example Answer: “PHP superglobals are predefined variables accessible everywhere:

  • $_GET: HTTP GET data
  • $_POST: HTTP POST data
  • $_SESSION: Session variables
  • $_COOKIE: Cookie values
  • $_SERVER: Server and environment information
  • $_FILES: File upload information
  • $GLOBALS: References to global scope variables
  • $_REQUEST: Combined GET, POST, and COOKIE data (avoid using)”

4. How do you handle errors in PHP?

Error handling demonstrates professionalism and code reliability.

Example Answer: “Multiple approaches for error handling:

// Try-catch for exceptions
try {
    $result = riskyOperation();
} catch (Exception $e) {
    error_log($e->getMessage());
    // Handle gracefully
}

// Custom error handler
set_error_handler(function($severity, $message, $file, $line) {
    throw new ErrorException($message, 0, $severity, $file, $line);
});

// Error reporting levels
error_reporting(E_ALL);
ini_set('display_errors', 0); // Production
ini_set('log_errors', 1);
```"

### 5. Explain object-oriented programming concepts in PHP.

OOP knowledge is essential for modern PHP development.

**Example Answer**: "PHP supports full OOP with classes, inheritance, encapsulation, and polymorphism:
```php
class Animal {
    protected $name;
    
    public function __construct($name) {
        $this->name = $name;
    }
    
    public function speak() {
        return 'Some sound';
    }
}

class Dog extends Animal {
    public function speak() {
        return $this->name . ' says Woof!';
    }
}

$dog = new Dog('Rex');
echo $dog->speak(); // Rex says Woof!
```"

### 6. What is the difference between abstract classes and interfaces?

This tests understanding of OOP design patterns.

**Example Answer**: 
- "Abstract classes can have both abstract and concrete methods, properties, and constructors. Classes can extend only one abstract class.
- Interfaces define method signatures only, no implementation. Classes can implement multiple interfaces.
```php
abstract class Shape {
    protected $color;
    abstract public function calculateArea();
    public function getColor() { return $this->color; }
}

interface Drawable {
    public function draw();
}
```"

### 7. How do you prevent SQL injection in PHP?

Security awareness is critical for web developers.

**Example Answer**: "Use prepared statements with bound parameters:
```php
// PDO with prepared statements
$stmt = $pdo->prepare('SELECT * FROM users WHERE email = ? AND status = ?');
$stmt->execute([$email, $status]);
$users = $stmt->fetchAll();

// MySQLi with prepared statements  
$stmt = $mysqli->prepare('INSERT INTO users (name, email) VALUES (?, ?)');
$stmt->bind_param('ss', $name, $email);
$stmt->execute();

Never use string concatenation for SQL queries.”

8. Explain PHP sessions and how they work.

Session management is fundamental for web applications.

Example Answer: “Sessions store user data across multiple pages. PHP creates unique session ID stored in cookie or URL:

session_start(); // Start session

$_SESSION['user_id'] = 123;
$_SESSION['username'] = 'john_doe';

// Access session data
if (isset($_SESSION['user_id'])) {
    echo 'Welcome back, ' . $_SESSION['username'];
}

// Destroy session
session_destroy();

Sessions are stored server-side, more secure than cookies.”

9. What are PHP namespaces and why use them?

Namespaces prevent naming conflicts in larger applications.

Example Answer: “Namespaces group related classes, functions, and constants to avoid naming conflicts:

namespace MyProject\Database;

class Connection {
    // Database connection logic
}

namespace MyProject\Http;

class Response {
    // HTTP response logic
}

// Usage
use MyProject\Database\Connection;
use MyProject\Http\Response;

$db = new Connection();
$response = new Response();

Essential for organizing code and using third-party libraries.”

10. How do you optimize PHP performance?

Performance optimization shows advanced understanding.

Example Answer: “Multiple optimization strategies:

  • Opcode caching: Use OPcache to cache compiled bytecode
  • Database optimization: Use proper indexing, avoid N+1 queries
  • Memory management: Unset variables, use generators for large datasets
  • Profiling: Use Xdebug or Blackfire to identify bottlenecks
  • HTTP caching: Implement proper cache headers
  • Code optimization: Avoid file_exists() in loops, use isset() instead of array_key_exists() php // Use generators for memory efficiency function getLargeDataset() { foreach ($data as $item) { yield $item; // Uses minimal memory } } ”

Modern PHP Concepts

For senior positions, explore these advanced topics:

Composer and Dependency Management

// composer.json
{
    "require": {
        "guzzlehttp/guzzle": "^7.0",
        "monolog/monolog": "^2.0"
    }
}

PHP 8+ Features

  • Union types: function process(string|int $value)
  • Named arguments: createUser(email: $email, name: $name)
  • Match expressions: More powerful switch statements
  • Attributes: Metadata for classes and methods
  • Laravel: Full-featured MVC framework
  • Symfony: Component-based framework
  • CodeIgniter: Lightweight and simple
  • Slim: Microframework for APIs

Practical Assessment Tips

When interviewing PHP candidates:

  1. Code review: Have candidates explain existing PHP code
  2. Problem solving: Present real-world scenarios requiring PHP solutions
  3. Security awareness: Test knowledge of common vulnerabilities
  4. Framework experience: Assess familiarity with relevant frameworks
  5. Best practices: Discuss PSR standards and coding conventions

Conclusion

PHP remains a cornerstone of web development, powering millions of websites worldwide. These interview questions help evaluate both fundamental understanding and practical application skills, enabling you to identify candidates who can contribute effectively to PHP-based projects.

The best PHP developers combine technical proficiency with understanding of web security, performance optimization, and modern development practices including version control, testing, and deployment strategies.

Use Interview Zen’s technical interview platform to create comprehensive PHP assessments and observe candidates’ problem-solving approaches in real-time during backend development interviews.