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Junior PHP Interview Questions

Curated Junior-level PHP interview questions for developers targeting junior positions. 30 questions available.

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PHP Interview Questions & Answers

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Welcome to our comprehensive collection of PHP interview questions and answers. This page contains expertly curated interview questions covering all aspects of PHP, from fundamental concepts to advanced topics. Whether you're preparing for an entry-level position or a senior role, you'll find questions tailored to your experience level.

Our PHP interview questions are designed to help you:

  • Understand core concepts and best practices in PHP
  • Prepare for technical interviews at all experience levels
  • Master both theoretical knowledge and practical application
  • Build confidence for your next PHP interview

Each question includes detailed answers and explanations to help you understand not just what the answer is, but why it's correct. We cover topics ranging from basic PHP concepts to advanced scenarios that you might encounter in senior-level interviews.

Use the filters below to find questions by difficulty level (Entry, Junior, Mid, Senior, Expert) or focus specifically on code challenges. Each question is carefully crafted to reflect real-world interview scenarios you'll encounter at top tech companies, startups, and MNCs.

Questions

30 questions
Q1:

What is PDO in PHP?

Junior

Answer

PDO is a database abstraction layer supporting prepared statements for secure, consistent DB access.
Quick Summary: PDO (PHP Data Objects) provides a unified interface for database access across different databases (MySQL, PostgreSQL, SQLite, etc.). Supports prepared statements (prevent SQL injection). OOP interface. Connection: new PDO("mysql:host=host;dbname=db", $user, $pass). Query: $stmt = $pdo->prepare("SELECT * FROM users WHERE id = ?"); $stmt->execute([$id]); $rows = $stmt->fetchAll().
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Q2:

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.
Quick Summary: SQL injection: attacker adds SQL code in user input that gets executed by the database. Prevention: always use prepared statements (parameterized queries): $stmt = $pdo->prepare("SELECT * FROM users WHERE email = ?"); $stmt->execute([$email]). Never concatenate user input into SQL strings. Also: validate/sanitize input, use least-privilege DB accounts, and escape output properly.
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Q3:

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.
Quick Summary: isset() checks if a variable is set (exists) and is not null. Returns true if the variable exists and has a non-null value. Returns false if undefined or null. Use before accessing potentially undefined variables (like $_GET["param"]) to avoid "undefined index" notices. isset($_GET["page"]) is the standard pattern for optional query parameters.
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Q4:

What is empty() in PHP?

Junior

Answer

empty() checks if a variable contains an empty or falsy value (0, "", null, false).
Quick Summary: empty() returns true if a variable is: not set, null, false, 0, 0.0, "0", "" (empty string), or [] (empty array). Different from isset() - empty also catches zero and empty strings. Use empty() when you want to check if a value is meaningless/absent. Caution: empty(0) is true, so use carefully with numeric values. isset($var) && $var !== "" is more explicit.
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Q5:

How does PHP handle errors?

Junior

Answer

PHP supports notices, warnings, and fatal errors. error_reporting and custom handlers manage behavior.
Quick Summary: PHP error handling: error_reporting() sets which errors to report. display_errors=On shows errors in browser (development only - never in production). error_log() writes to log file. set_error_handler() installs a custom error handler. try/catch for exceptions. set_exception_handler() for uncaught exceptions. In production: log errors, never display them to users. Use a logging library like Monolog.
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Q6:

What is the purpose of php.ini?

Junior

Answer

php.ini configures PHP settings such as memory limits, error reporting, sessions, and upload sizes.
Quick Summary: php.ini is the main config file for PHP. Controls memory limit, upload size, error reporting, session settings, timezone, and extension loading. Located at /etc/php/php.ini (Linux). PHP reads it on startup. Changes need a server restart. Use phpinfo() to see what values are active. Each project can override with per-directory .user.ini files.
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Q7:

What is file handling in PHP?

Junior

Answer

PHP supports reading, writing, appending, and deleting files for logs, exports, or resources.
Quick Summary: PHP file handling: fopen() opens a file with a mode (r for read, w for write, a for append). fread(), fgets() for reading. fwrite() for writing. fclose() to close. file_get_contents() reads an entire file as a string. file_put_contents() writes a string to a file. Always close files after use and check for errors.
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Q8:

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.
Quick Summary: Single quotes treat content as literal text - no variable interpolation, minimal escape sequences. Double quotes parse variables and escape sequences like newlines. Single quotes are slightly faster. Use double quotes when you need variables embedded in the string. Use single quotes for static strings. Both are valid, consistency matters most.
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Q9:

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.
Quick Summary: header() sends raw HTTP headers to the browser. Must be called before any output. Used for: redirects (Location header), setting content type (Content-Type: application/json), caching headers (Cache-Control), file download prompts (Content-Disposition). Always call exit after a redirect header to stop script execution immediately.
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Q10:

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.
Quick Summary: json_encode() converts PHP arrays/objects into a JSON string. json_decode() parses a JSON string into PHP objects or arrays (pass true as second arg for arrays). Returns null on failure. Check json_last_error() for error details. Used heavily for API responses. JSON is the standard data format for REST APIs and config files.
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Q11:

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.
Quick Summary: GET sends data in the URL query string - visible, bookmarkable, cached, length-limited. Use for read-only operations like search. POST sends data in the request body - not visible in URL, supports large payloads and file uploads. Use for write operations like form submission or login. Both need validation before use.
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Q12:

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.
Quick Summary: Type juggling is PHP automatically converting types during comparisons. Using == (loose comparison), "1" equals 1, and empty string equals false. This can cause bugs. Always use === (strict comparison) which checks both value and type. PHP 8 fixed many loose comparison inconsistencies. Explicit type casting or strict_types declaration helps avoid juggling issues.
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Q13:

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.
Quick Summary: filter_var() validates and sanitizes data. FILTER_VALIDATE_EMAIL checks email format. FILTER_SANITIZE_STRING removes tags. FILTER_VALIDATE_INT validates integers. FILTER_VALIDATE_URL checks URL format. Cleaner than writing regex manually. Use for validating user input at entry points. Part of PHP built-ins, no extra library needed.
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Q14:

What are magic constants in PHP?

Junior

Answer

Magic constants like __FILE__, __LINE__, and __DIR__ provide dynamic information depending on where they are used.
Quick Summary: Magic constants change value depending on where they are used. __LINE__ gives current line number, __FILE__ gives full file path, __DIR__ gives directory path, __CLASS__ gives class name, __METHOD__ gives method name, __NAMESPACE__ gives current namespace. Useful for debugging, logging, and building autoloaders. Always available without any import.
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Q15:

What are magic methods in PHP?

Junior

Answer

Magic methods (__construct, __destruct, __get, __set, __toString) provide special object behaviors and overloading capabilities.
Quick Summary: Magic methods are automatically called by PHP in certain situations. __construct runs on object creation, __destruct on cleanup, __get and __set handle undefined property access, __call handles undefined method calls, __toString is called when object is cast to string, __invoke when object is called as a function. They give objects custom behavior for built-in PHP operations.
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Q16:

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.
Quick Summary: include_once and require_once track which files have been loaded and skip re-loading them. This prevents errors from redeclaring classes or functions. require_once causes a fatal error if the file is not found; include_once shows only a warning. Use require_once for class files and libraries where double-loading would cause redeclaration errors.
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Q17:

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.
Quick Summary: session_start() must be called at the top of every page using sessions, before any output. It reads the session ID from the cookie, loads saved session data, and makes the session array available. Without it the session array does not exist. After starting a session you can store and read user data across multiple page requests.
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Q18:

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.
Quick Summary: Output buffering stores PHP output in memory instead of sending it directly to the browser. ob_start() starts buffering. ob_get_clean() returns the buffered content and clears it. Benefits: lets you set headers after output begins, manipulate output before sending, compress responses. Frameworks use it to wrap layout templates around content.
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Q19:

What are PHP traits?

Junior

Answer

Traits allow code reuse by combining methods from multiple sources without using inheritance.
Quick Summary: Traits enable code reuse in PHP single-inheritance. A trait defines methods that can be mixed into any class. A class can use multiple traits. Unlike interfaces, traits provide actual implementations. Use traits for shared behaviors like logging, timestamps, or soft-delete across unrelated classes. Conflicts between traits are resolved with the insteadof and as keywords.
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Q20:

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.
Quick Summary: public: accessible from anywhere (outside the class, subclasses, same class). protected: accessible within the class and its subclasses. private: accessible only within the same class - not even subclasses. Apply to properties and methods. Default is public in PHP. Use private for internal implementation, protected for extensible behavior, public for the class's interface.
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Q21:

How does PHP handle exceptions?

Junior

Answer

Exceptions are handled with try-catch blocks, allowing structured error management.
Quick Summary: PHP exceptions: throw new Exception("message") or throw new CustomException(). Catch with try-catch: try { riskyCode(); } catch (SpecificException $e) { handle(); } catch (Exception $e) { generic(); } finally { cleanup(); }. Multiple catch blocks handle different types. PHP 8 allows catching multiple types: catch (IOException|DatabaseException $e). Create custom exceptions by extending Exception.
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Q22:

What is the purpose of namespaces in PHP?

Junior

Answer

Namespaces avoid naming conflicts and organize large applications by grouping related classes.
Quick Summary: Namespaces prevent name collisions between classes, functions, and constants from different libraries. namespace App\\Models; declares the namespace. Use other namespaces: use App\\Services\\UserService;. Access without use: new App\\Models\\User(). PHP files with the same class name can coexist in different namespaces. Required for Composer autoloading and modern OOP PHP development.
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Q23:

What is autoloading in PHP?

Junior

Answer

Autoloading automatically loads classes when first referenced, commonly via spl_autoload_register or Composer.
Quick Summary: Autoloading automatically loads class files when a class is first used. No manual require needed. PHP has spl_autoload_register() for custom autoloaders. Composer generates an optimized PSR-4 autoloader that maps namespace prefixes to directory paths. Just include vendor/autoload.php once and all classes load automatically. Essential for any modern PHP project.
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Q24:

What is Composer in PHP?

Junior

Answer

Composer is a dependency manager that installs, updates, and autoloads external PHP libraries.
Quick Summary: Composer is PHP's dependency manager. Define project dependencies in composer.json. composer install downloads packages to vendor/. composer update upgrades to latest versions. composer require adds new packages. It also generates the autoloader at vendor/autoload.php. PSR-4 autoloading maps namespaces to directories. Standard tool for all modern PHP projects.
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Q25:

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.
Quick Summary: For arrays, == checks if both arrays have the same key-value pairs (order does not matter, types coerced). === checks same key-value pairs in same order with same types. [1, 2] == ["0" => 1, "1" => 2] is true. [1, 2] === [2, 1] is false. Always use === for arrays when order or types matter to avoid surprises.
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Q26:

What is the purpose of htmlentities()?

Junior

Answer

htmlentities() converts all applicable characters to HTML entities, preventing XSS attacks.
Quick Summary: htmlentities() converts special characters to HTML entities to prevent XSS attacks. < becomes < > becomes > and & becomes &. Always encode user-provided data before outputting it in HTML. htmlspecialchars() is the lighter version that only converts the five critical characters. Use ENT_QUOTES flag to also encode single quotes.
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Q27:

How do you connect PHP with MySQL using PDO?

Junior

Answer

PDO connects PHP to MySQL using DSN strings and supports secure prepared statements.
Quick Summary: PDO connection: create a new PDO object with DSN string (mysql:host=localhost;dbname=mydb), username, and password. Set error mode to exceptions for proper error handling. Use prepare() to create a prepared statement, bind parameters, then execute(). PDO works with multiple databases. Store connection in a singleton or use a DI container to avoid opening multiple connections.
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Q28:

What is the difference between mysqli and PDO?

Junior

Answer

mysqli supports only MySQL; PDO supports multiple databases and provides better abstraction.
Quick Summary: mysqli is MySQL-specific. PDO supports 12+ database drivers (MySQL, PostgreSQL, SQLite, etc). PDO uses a consistent API regardless of database. mysqli has both procedural and OOP API. PDO uses only OOP. Both support prepared statements. Choose PDO for portability and consistency. Use mysqli only when you need MySQL-specific features like multiple result sets.
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Q29:

What is a callback function in PHP?

Junior

Answer

A callback is a function passed as an argument and executed inside another function.
Quick Summary: A callback is a function passed as an argument to another function. PHP accepts closures, arrow functions, function name strings, or array with object+method. Used with array_map(), array_filter(), usort(), etc. Callbacks let you customize behavior without changing the function. Closures can capture outer variables using the use keyword.
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Q30:

What is the purpose of phpinfo()?

Junior

Answer

phpinfo() displays PHP configuration, extensions, environment variables, and server settings.
Quick Summary: phpinfo() outputs a full HTML page showing PHP version, build info, loaded extensions, ini settings, environment variables, and HTTP request details. Extremely useful for debugging config issues. Critical security risk in production - never expose phpinfo() publicly. Use it locally or in dev only, then remove it. One of the first tools to check when debugging a PHP setup.
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People Also Ask - Related PHP Questions

What is PDO in PHP?
What is SQL injection and how do you prevent it in PHP?
What is the use of isset() in PHP?
What is empty() in PHP?
How does PHP handle errors?
What is the purpose of php.ini?
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