PHP Interview Questions

Q1:

What is PHP and where is it commonly used?

Entry

Answer

PHP is a server-side scripting language used to build dynamic websites. It runs on the server, generates HTML, and integrates well with databases like MySQL.

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Q2:

What does server-side scripting mean in PHP?

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Answer

Server-side scripting means PHP code executes on the server and outputs HTML to the client. The browser never sees the PHP code.

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Q3:

What is the difference between echo and print?

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Answer

echo outputs text faster and accepts multiple parameters. print returns 1 and can be used in expressions.

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Q4:

What are PHP variables and how are they declared?

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Answer

PHP variables start with $ and are dynamically typed, meaning type is assigned at runtime.

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Q5:

What are PHP data types?

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Answer

PHP supports scalars (string, int, float, bool), compound types (array, object), and special types (null, resource).

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Q6:

What is an associative array in PHP?

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Answer

An associative array uses named keys instead of numeric indexes. Useful for key-value mappings.

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Q7:

What is the difference between == and === in PHP?

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Answer

== compares values with type juggling. === compares both value and type.

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Q8:

What are PHP superglobal variables?

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Answer

Superglobals ($_GET, $_POST, $_SESSION, $_COOKIE, $_SERVER) are built-in arrays available globally.

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Q9:

What is the role of $_GET and $_POST?

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Answer

$_GET gets URL parameters. $_POST retrieves data sent through the request body, often for forms.

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Q10:

What is a session in PHP?

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Answer

Sessions store user-specific data on the server and persist across requests.

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Q11:

What is a cookie in PHP?

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Answer

A cookie stores small data in the user’s browser for preferences or authentication.

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Q12:

What is the include statement used for?

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Answer

include loads and executes a file at runtime to reuse common code.

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Q13:

What is the difference between include and require?

Entry

Answer

include gives a warning on failure; require stops execution with a fatal error.

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Q14:

What is form handling in PHP?

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Answer

Form handling retrieves data using $_GET or $_POST and includes validation and sanitization steps.

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Q15:

What is the purpose of htmlspecialchars()?

Entry

Answer

htmlspecialchars() converts < > & characters into HTML entities to prevent XSS attacks.

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Q16:

What is PDO in PHP?

Junior

Answer

PDO is a database abstraction layer supporting prepared statements for secure, consistent DB access.

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Q17:

What is SQL injection and how do you prevent it in PHP?

Junior

Answer

SQL injection manipulates queries using malicious input. Prevent using prepared statements with bound parameters.

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Q18:

What is the use of isset() in PHP?

Junior

Answer

isset() checks whether a variable exists and is not null. Commonly used for request validation.

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Q19:

What is empty() in PHP?

Junior

Answer

empty() checks if a variable contains an empty or falsy value (0, "", null, false).

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Q20:

How does PHP handle errors?

Junior

Answer

PHP supports notices, warnings, and fatal errors. error_reporting and custom handlers manage behavior.

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Q21:

What is the purpose of php.ini?

Junior

Answer

php.ini configures PHP settings such as memory limits, error reporting, sessions, and upload sizes.

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Q22:

What is file handling in PHP?

Junior

Answer

PHP supports reading, writing, appending, and deleting files for logs, exports, or resources.

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Q23:

What is the difference between single and double quotes in PHP?

Junior

Answer

Single quotes output text literally; double quotes parse variables and escape sequences.

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Q24:

What is the purpose of the header() function?

Junior

Answer

header() sends raw HTTP headers for redirects, JSON output, or content-type changes and must run before output.

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Q25:

What is JSON encoding and decoding in PHP?

Junior

Answer

json_encode() converts arrays/objects to JSON; json_decode() converts JSON strings back to PHP structures.

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Q26:

What is the difference between GET and POST methods in PHP?

Junior

Answer

GET appends data to the URL and is limited in size. POST sends data in the request body, supports larger payloads, and is more secure for sensitive data.

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Q27:

What is PHP’s type juggling and how does it affect comparisons?

Junior

Answer

PHP automatically converts data types during comparisons. This may cause unexpected results unless strict (===) comparison is used.

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Q28:

What is the purpose of filter_var() in PHP?

Junior

Answer

filter_var() validates and sanitizes data using built-in filters for email, URL, integers, and more.

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Q29:

What are magic constants in PHP?

Junior

Answer

Magic constants like __FILE__, __LINE__, and __DIR__ provide dynamic information depending on where they are used.

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Q30:

What are magic methods in PHP?

Junior

Answer

Magic methods (__construct, __destruct, __get, __set, __toString) provide special object behaviors and overloading capabilities.

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Q31:

What is the difference between include_once and require_once?

Junior

Answer

Both include a file only once. include_once gives a warning on failure, require_once causes a fatal error.

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Q32:

What is the purpose of session_start()?

Junior

Answer

session_start() begins or resumes a session and must be called before any output is sent.

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Q33:

What is output buffering in PHP?

Junior

Answer

Output buffering stores output in a buffer so headers can be modified and output order can be controlled.

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Q34:

What are PHP traits?

Junior

Answer

Traits allow code reuse by combining methods from multiple sources without using inheritance.

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Q35:

What is the difference between public, private, and protected?

Junior

Answer

public is accessible everywhere, private only inside the class, protected inside the class and subclasses.

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Q36:

How does PHP handle exceptions?

Junior

Answer

Exceptions are handled with try-catch blocks, allowing structured error management.

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Q37:

What is the purpose of namespaces in PHP?

Junior

Answer

Namespaces avoid naming conflicts and organize large applications by grouping related classes.

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Q38:

What is autoloading in PHP?

Junior

Answer

Autoloading automatically loads classes when first referenced, commonly via spl_autoload_register or Composer.

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Q39:

What is Composer in PHP?

Junior

Answer

Composer is a dependency manager that installs, updates, and autoloads external PHP libraries.

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Q40:

What is the difference between == and === when comparing arrays?

Junior

Answer

== compares arrays by key-value pairs ignoring type; === checks type, order, and structure strictly.

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Q41:

What is the purpose of htmlentities()?

Junior

Answer

htmlentities() converts all applicable characters to HTML entities, preventing XSS attacks.

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Q42:

How do you connect PHP with MySQL using PDO?

Junior

Answer

PDO connects PHP to MySQL using DSN strings and supports secure prepared statements.

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Q43:

What is the difference between mysqli and PDO?

Junior

Answer

mysqli supports only MySQL; PDO supports multiple databases and provides better abstraction.

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Q44:

What is a callback function in PHP?

Junior

Answer

A callback is a function passed as an argument and executed inside another function.

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Q45:

What is the purpose of phpinfo()?

Junior

Answer

phpinfo() displays PHP configuration, extensions, environment variables, and server settings.

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Q46:

What is the difference between require, include, require_once, and include_once in real project scenarios?

Mid

Answer

require stops execution if the file is missing, include only warns. require_once and include_once prevent duplicate loading. In real projects, require_once is used for config, autoloaders, and shared logic.

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Q47:

What is the role of PHP’s garbage collection mechanism?

Mid

Answer

PHP uses reference counting and a cyclic garbage collector to free memory. It detects circular references and prevents memory leaks.

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Q48:

What is the difference between session storage and token-based authentication?

Mid

Answer

Sessions store user state on the server. Token-based auth (like JWT) stores authentication data client-side and is stateless, ideal for APIs and distributed systems.

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Q49:

How does PHP handle file uploads and what security concerns exist?

Mid

Answer

PHP handles uploads via $_FILES. Security concerns: MIME validation, size limits, avoiding directory traversal, storing outside webroot, renaming files, and sanitizing filenames.

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Q50:

What is output escaping and why is it important in PHP apps?

Mid

Answer

Output escaping prevents XSS by converting special characters using htmlspecialchars() or htmlentities().

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Q51:

How do prepared statements work in PDO?

Mid

Answer

Prepared statements compile SQL once and bind parameters later. They prevent SQL injection and improve repeated query performance.

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Q52:

What is SPL in PHP and why use it?

Mid

Answer

SPL provides interfaces, iterators, and data structures like SplStack and SplQueue to write efficient, reusable code.

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Q53:

What PHP design patterns are commonly used at mid-level?

Mid

Answer

Common patterns: Singleton, Factory, Strategy, Decorator, Repository. These help structure scalable, maintainable code.

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Q54:

What is the difference between static and non-static methods?

Mid

Answer

Static methods belong to the class and do not need an instance. Non-static methods operate on object properties. Static overuse reduces flexibility.

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Q55:

What is dependency injection in PHP?

Mid

Answer

Dependency injection provides dependencies from outside the class. It improves testability and decoupling. Frameworks like Laravel use DI containers.

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Q56:

How does PHP handle JSON parsing errors?

Mid

Answer

json_decode() returns null on failure and json_last_error() provides the error code. Proper handling is required when interacting with APIs.

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Q57:

What is the difference between abstract classes and interfaces?

Mid

Answer

Abstract classes contain shared logic plus abstract methods. Interfaces define method signatures. Interfaces enforce capabilities; abstract classes share implementation.

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Q58:

How do you secure sensitive configuration values in PHP applications?

Mid

Answer

Use environment variables, encrypted config files, or framework vaults. Never store secrets in code or public repos.

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Q59:

What is PHP’s password_hash() and why is it recommended?

Mid

Answer

password_hash() uses strong algorithms like bcrypt/argon2 with auto-salting, removing the need for manual security handling.

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Q60:

What is rate limiting and how can it be implemented in PHP?

Mid

Answer

Rate limiting restricts requests per user/IP using Redis counters, middleware, or token bucket algorithms. Prevents abuse.

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Q61:

What is CSRF and how do you prevent it in PHP apps?

Mid

Answer

CSRF tricks users into unintended actions. Prevent using CSRF tokens, SameSite cookies, and referer validation.

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Q62:

What is the difference between file_get_contents and curl for HTTP requests?

Mid

Answer

file_get_contents() handles simple GET/POST. curl supports headers, auth, timeouts, redirects, and complex HTTP tasks. Preferred for APIs.

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Q63:

What is autoloading (PSR-4) and how does it improve structure?

Mid

Answer

PSR-4 maps namespaces to folders. Composer autoloading eliminates manual include calls and keeps code organized.

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Q64:

What is the purpose of htaccess in PHP applications?

Mid

Answer

.htaccess configures Apache: URL rewriting, security rules, redirects, and access control per directory.

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Q65:

How does PHP handle timezones and why is it important?

Mid

Answer

PHP uses date_default_timezone_set() or php.ini. Correct timezone handling prevents scheduling bugs and inconsistent timestamps.

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Q66:

How does PHP-FPM improve performance compared to traditional mod_php?

Senior

Answer

PHP-FPM manages pools of worker processes independently from the web server. It supports scaling, process recycling, chroot isolation, and efficient load balancing. It is faster and more secure compared to mod_php.

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Q67:

How does PHP handle memory management internally?

Senior

Answer

PHP uses reference counting to track variables. The garbage collector removes cyclic references. Large arrays or extensions can exhaust memory if not properly cleaned.

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Q68:

What are PHP Opcache benefits and what problems does it solve?

Senior

Answer

Opcache caches precompiled PHP bytecode in shared memory, eliminating the need to recompile scripts on each request. It reduces CPU usage and improves response time.

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Q69:

How do you diagnose performance issues using Xdebug or Blackfire?

Senior

Answer

Profilers like Xdebug and Blackfire track execution time, memory usage, and call graphs to identify bottlenecks in loops, database calls, and slow functions.

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Q70:

What is autoloading with Composer and how does classmap optimization work?

Senior

Answer

Composer autoloads classes based on PSR-4 or classmap. Classmap optimization precomputes file paths, reducing filesystem lookups and improving performance.

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Q71:

What is the difference between synchronous and asynchronous PHP execution?

Senior

Answer

Traditional PHP runs synchronously. Async PHP (ReactPHP, Swoole, Amp) uses event loops to handle concurrent non-blocking I or O for real-time performance.

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Q72:

How do you design a scalable session management system in PHP?

Senior

Answer

Use Redis or Memcached for shared sessions. Avoid file-based sessions. Keep session data minimal and avoid sticky sessions unless necessary.

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Q73:

What is the purpose of PHP’s SPL Iterators and why are they useful?

Senior

Answer

SPL Iterators allow efficient traversal of large datasets with objects like DirectoryIterator, ArrayIterator, and RecursiveIterator.

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Q74:

How do you protect PHP applications against timing attacks?

Senior

Answer

Use constant-time comparison functions like hash_equals() to prevent timing-based information leaks.

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Q75:

How do you scale PHP applications horizontally?

Senior

Answer

Use load balancers, stateless design, shared cache layers, distributed sessions, and PHP-FPM behind Nginx or Apache.

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Q76:

How does PHP handle multi-byte strings and why use mbstring?

Senior

Answer

Native PHP functions treat strings as bytes. mbstring ensures correct handling of UTF-8 multibyte characters to avoid corrupted text.

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Q77:

What is the importance of immutability in PHP value objects?

Senior

Answer

Immutable objects prevent unexpected state changes and improve debugging. Useful in domain-driven design.

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Q78:

How do you enforce coding standards in large PHP applications?

Senior

Answer

Use tools like PHP_CodeSniffer, PHP-CS-Fixer, and PHPMD. Integrate them into CI pipelines to enforce PSR-12.

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Q79:

What is PHP’s Reflection API and when should it be used?

Senior

Answer

Reflection inspects classes, methods, and parameters. Useful for dependency injection containers and ORMs.

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Q80:

How do you secure PHP apps against SQL injection beyond prepared statements?

Senior

Answer

Use parameterized queries, avoid dynamic SQL, validate input types, and use secure ORM query builders.

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Q81:

What are generics in PHP 8.2+ via static analysis tools?

Senior

Answer

PHP lacks runtime generics, but tools like Psalm and PHPStan add generic annotations for type-safe collections.

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Q82:

What is the difference between DI container and service locator pattern?

Senior

Answer

DI containers inject dependencies automatically. Service locator hides dependencies and reduces testability.

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Q83:

How do you manage configuration in multi-environment PHP systems?

Senior

Answer

Use .env files, secrets managers, or environment-specific configs. Avoid hardcoding credentials.

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Q84:

How does Laravel handle request lifecycle compared to raw PHP?

Senior

Answer

Laravel handles requests via Kernel, middleware, router, controller, and response pipeline, unlike raw PHP which executes scripts directly.

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Q85:

What is the role of event loops in async PHP frameworks?

Senior

Answer

Event loops manage asynchronous non-blocking operations for sockets, timers, and file I or O.

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Q86:

How do you optimize PHP-FPM for high traffic?

Senior

Answer

Tune pm.max_children, pm.start_servers, and pm.max_requests. Monitor slow logs and adjust based on traffic patterns.

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Q87:

What is opcode preloading in PHP 7.4+?

Senior

Answer

Opcode preloading loads classes and functions into Opcache on server startup to eliminate cold starts.

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Q88:

How do you implement message queues in PHP?

Senior

Answer

Use RabbitMQ, Kafka, Redis streams, or AWS SQS for async job processing and background tasks.

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Q89:

How does PHP handle large file processing efficiently?

Senior

Answer

Use streaming functions like fopen and fgets, and generators to avoid loading entire files into memory.

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Q90:

What is a generator in PHP and why is it memory-efficient?

Senior

Answer

Generators return values lazily using yield, reducing memory usage for large datasets.

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Q91:

How does PHP interact with Redis or Memcached for caching?

Senior

Answer

PHP extensions allow storing cached data, sessions, and queues using Redis or Memcached.

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Q92:

What are PSR standards and why are they important?

Senior

Answer

PSR standards define coding rules and interfaces for interoperability, including PSR-1, PSR-12, PSR-4, and PSR-7.

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Q93:

How do you protect PHP APIs against brute-force attacks?

Senior

Answer

Use rate limits, IP blocking, CAPTCHAs, and throttling mechanisms.

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Q94:

What is Domain-Driven Design (DDD) and how does it apply to PHP?

Senior

Answer

DDD structures software around domain models and uses value objects, aggregates, and repositories. Works well with Symfony and Laravel.

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Q95:

Why do large PHP monoliths eventually require modularization?

Senior

Answer

Monoliths become hard to maintain. Modularization or microservices improve scalability, deployment, and testability.

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Q96:

How do you architect PHP applications to handle millions of requests per day without horizontal bottlenecks?

Expert

Answer

Use stateless architecture, distributed cache, Redis-based shared sessions, CDN, optimized PHP-FPM pools, async queues, and load balancing with Nginx. Combine read replicas, DB sharding, and Opcache preloading to eliminate CPU and I/O bottlenecks.

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Q97:

How do you tune PHP-FPM for ultra-high concurrency workloads?

Expert

Answer

Tune pm settings based on memory per worker and CPU. Use dynamic or ondemand mode, set pm.max_children appropriately, and use pm.max_requests to recycle workers. Analyze slow logs and worker queue saturation for optimal concurrency.

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Q98:

How does Opcache internals handle shared memory, and why are resets expensive?

Expert

Answer

Opcache stores precompiled opcodes in shared memory. Resetting clears the cache, forcing recompilation of all scripts, causing CPU spikes and latency. Production systems avoid frequent resets except during deployments.

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Q99:

What are the architectural benefits of using async PHP frameworks like Swoole or ReactPHP?

Expert

Answer

Async frameworks replace the synchronous model with event loops, eliminating process-per-request overhead. They enable WebSockets, microservices, real-time APIs, and long-running processes with minimal latency.

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Q100:

How do you implement advanced caching strategy using layered caches?

Expert

Answer

Use opcode cache for compilation, APCu as local memory cache for microsecond access, and Redis or Memcached as distributed cache. Multi-layer caching minimizes DB hits and achieves very high cache-hit ratios.

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Q101:

How does PHP’s JIT impact performance and what workloads benefit?

Expert

Answer

JIT compiles specific code paths to machine code. It benefits CPU-heavy operations such as math, image processing, and loops. Web applications benefit minimally due to I/O-bound workloads.

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Q102:

How do you design scalable microservices using PHP while avoiding tight coupling?

Expert

Answer

Use message brokers, REST or GraphQL, async event buses, service discovery, and strict domain boundaries. Avoid shared databases. Use DTOs and versioned APIs to prevent coupling.

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Q103:

How do advanced DI containers optimize object instantiation?

Expert

Answer

DI containers use reflection caching, lazy loading, compiled containers, and autowiring rules. They reduce runtime reflection calls and speed up object graph construction.

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Q104:

What causes memory leaks in long-running PHP processes and how to prevent them?

Expert

Answer

Leaks arise from circular references, static variables, unclosed resources, and growing arrays. Prevent using GC cycles, avoiding static state, releasing resources, and recycling workers via pm.max_requests.

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Q105:

Why is immutability essential in transactional and multi-threaded systems?

Expert

Answer

Immutable objects eliminate race conditions and ensure predictable behavior. They are crucial in financial systems and async environments where shared state causes corruption.

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Q106:

How do you implement domain events and event sourcing in PHP?

Expert

Answer

Record state changes as immutable events, store them in an event store, process projections to build read models, and use event buses for propagation. Replay events to rebuild aggregates.

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Q107:

How do you structure extremely large PHP applications using modular monolith patterns?

Expert

Answer

Group code into domain modules with strict boundaries enforced via namespaces and DI. Avoid cross-module dependencies. Use domain layers with well-defined APIs.

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Q108:

How do you optimize SQL access layers for high-traffic PHP applications?

Expert

Answer

Use read replicas, query batching, write-behind caching, persistent connections, and prepared statements. Eliminate N+1 queries with eager loading and caching.

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Q109:

How do you build zero-downtime deployments for PHP applications?

Expert

Answer

Use blue-green or rolling deploys, warm Opcache preload, backward-compatible DB migrations, and health checks with circuit breakers during traffic switching.

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Q110:

How do you detect and mitigate race conditions in distributed PHP workflows?

Expert

Answer

Use atomic operations, file locks, DB advisory locks, Redis RedLock, and idempotent writes. Implement retry logic and conflict detection for safe distributed workflows.

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Top PHP Interview Questions