100 PHP interview questions and answers

Certainly! Here are 100 PHP interview questions and answers. These cover a wide range of topics from basic to advanced PHP concepts.

Basic Questions

1. What is PHP?
   • Answer: PHP stands for “Hypertext Preprocessor.” It is a widely-used open-source server-side scripting language especially suited for web development and can be embedded into HTML.
2. What are the common uses of PHP?
   • Answer: PHP is commonly used to create dynamic web pages, develop server-side applications, manage databases, and handle sessions and cookies.
3. How do you declare a variable in PHP?
   • Answer: Variables in PHP are declared using the $ sign followed by the variable name. For example, $variable = 'value';.
4. What are the different types of data types in PHP?
   • Answer: PHP supports several data types including integers, floats, strings, arrays, objects, resources, and NULL.
5. What is the difference between echo and print in PHP?
   • Answer: Both echo and print are used to output data to the screen. echo can take multiple parameters, while print can only take one and always returns 1.
6. How can you concatenate strings in PHP?
   • Answer: Strings in PHP can be concatenated using the . operator. For example, $string1 . $string2.
7. What are constants in PHP and how are they defined?
   • Answer: Constants are defined using the define() function and cannot be changed once set. For example, define('CONSTANT_NAME', 'value');.
8. What is a PHP session?
   • Answer: A session is a way to store information (in variables) to be used across multiple pages. Unlike cookies, the information is not stored on the user’s computer.
9. What is the difference between include and require in PHP?
   • Answer: Both include and require are used to include files in PHP. The difference is that require will produce a fatal error and stop the script if the file is not found, while include will only produce a warning and the script will continue.
10. How do you connect to a MySQL database using PHP?
    • Answer: You can connect to a MySQL database using the mysqli_connect() function. Example: $connection = mysqli_connect('hostname', 'username', 'password', 'database');.

Intermediate Questions

11. What is an associative array in PHP?
    • Answer: An associative array is an array where each key has a specific value assigned to it. For example, $array = array('key1' => 'value1', 'key2' => 'value2');.
12. How can you check the data type of a variable in PHP?
    • Answer: You can use functions like gettype(), is_int(), is_string(), etc., to check the data type of a variable.
13. What are PHP magic methods?
    • Answer: Magic methods are special methods that begin with double underscores (__). Common magic methods include __construct(), __destruct(), __call(), __get(), __set(), and __toString().
14. What is the difference between == and === in PHP?
    • Answer: == checks for equality of value without considering data type, whereas === checks for equality of both value and data type.
15. What is the purpose of the final keyword in PHP?
    • Answer: The final keyword prevents a class from being inherited or a method from being overridden.
16. What are traits in PHP?
    • Answer: Traits are a mechanism for code reuse in single inheritance languages like PHP. A trait is intended to reduce some limitations of single inheritance by enabling a developer to reuse sets of methods freely in several independent classes.
17. How do you handle errors in PHP?
    • Answer: Errors in PHP can be handled using custom error handlers set by set_error_handler(), and exceptions can be handled using try-catch blocks.
18. What is the difference between GET and POST methods in PHP?
    • Answer: GET requests data from a specified resource and appends the data to the URL. POST submits data to be processed to a specified resource and does not append data to the URL, which is more secure for sensitive information.
19. How do you upload a file in PHP?
    • Answer: You can upload a file in PHP using the $_FILES superglobal. The file upload process involves creating an HTML form with enctype="multipart/form-data", then handling the file upload in the PHP script using move_uploaded_file().
20. What is a PHP closure?
    • Answer: A closure is an anonymous function that can capture variables from its surrounding scope. Example: $closure = function($name) { return "Hello, $name"; };.

Advanced Questions

21. What is PDO in PHP?
    • Answer: PDO (PHP Data Objects) is a database access layer providing a uniform method of access to multiple databases. It does not provide a database abstraction but a data-access abstraction.
22. What is the difference between include_once and require_once in PHP?
    • Answer: include_once and require_once include and evaluate the specified file during the execution of the script, but will do so only if it has not been included or required before. require_once will cause a fatal error if the file is not found, while include_once will only emit a warning.
23. How can you implement a singleton design pattern in PHP?
    • Answer: A singleton pattern ensures a class has only one instance and provides a global point of access to it. Example implementation:

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

24. What are namespaces in PHP?
    Answer: Namespaces are a way of encapsulating items to avoid naming conflicts. They allow the same name to be used for more than one item. Example: namespace MyNamespace;.
25. What is the difference between unserialize() and json_decode() in PHP?
    • Answer: unserialize() takes a serialized string and converts it back into a PHP value, whereas json_decode() takes a JSON-encoded string and converts it into a PHP value.
26. How do you implement inheritance in PHP?
    • Answer: Inheritance is implemented by creating a parent class and using the extends keyword for child classes. Example:

<?php
class ParentClass {
   public function parentMethod() {
     return "This is a parent method";
   }
}
class ChildClass extends ParentClass {
   public function childMethod() {
     return "This is a child method";
   }
}

27. What is the difference between abstract classes and interfaces in PHP?
    Answer: Abstract classes can have implemented methods and properties, and child classes must implement any abstract methods. Interfaces can only have method declarations, and implementing classes must implement all methods of the interface.
28. How do you secure a PHP application?
    • Answer: Security measures include input validation and sanitization, using prepared statements and parameterized queries to prevent SQL injection, proper error handling, session management, and using HTTPS.
29. What is a PSR in PHP?
    • Answer: PSR (PHP Standards Recommendation) are coding standards and guidelines published by the PHP-FIG (Framework Interoperability Group). Examples include PSR-1 (Basic Coding Standard), PSR-2 (Coding Style Guide), and PSR-4 (Autoloading Standard).
30. How do you use Composer in PHP?
    • Answer: Composer is a dependency manager for PHP. You can install it, create a composer.json file with your project dependencies, and run composer install to download and manage these dependencies.

Database and SQL Questions

31. How do you fetch data from a MySQL database in PHP?
    • Answer: You can fetch data using mysqli_query() or PDO. Example using mysqli:

<?php

$connection = mysqli_connect('hostname', 'username', 'password', 'database');
$query = "SELECT * FROM table";
$result = mysqli_query($connection, $query);
while ($row = mysqli_fetch_assoc($result)) {
    echo $row['column_name'];
}

32. What is the difference between mysqli_fetch_assoc() and mysqli_fetch_array()?
    • Answer: mysqli_fetch_assoc() fetches a result row as an associative array, while mysqli_fetch_array() fetches a result row as both an associative array and a numeric array.
33. How do you prevent SQL injection in PHP?
    • Answer: Use prepared statements and parameterized queries with mysqli or PDO. Example using PDO:

<?php
$pdo = new PDO('mysql:host=hostname;dbname=database', 'username', 'password');
$stmt = $pdo->prepare('SELECT * FROM table WHERE column = :value');
$stmt->execute(['value' => $inputValue]);

34. How do you handle transactions in PHP using PDO?
    • Answer: Transactions can be handled using beginTransaction(), commit(), and rollBack() methods. Example:

<?php
try {
    $pdo->beginTransaction();
    // Execute multiple queries
    $pdo->commit();
} catch (Exception $e) {
    $pdo->rollBack();
    echo "Failed: " . $e->getMessage();
}

35. What are joins in SQL and how do you use them in PHP?
    • Answer: Joins are used to combine rows from two or more tables based on a related column. Example of an inner join:

<?php
$query = "SELECT users.name, orders.amount FROM users INNER JOIN orders ON users.id = orders.user_id";
$result = mysqli_query($connection, $query);

36. How do you count the number of rows in a table using PHP?
    • Answer: Use the SQL COUNT function. Example:

<?php
$query = "SELECT COUNT(*) as total FROM table";
$result = mysqli_query($connection, $query);
$row = mysqli_fetch_assoc($result);
echo $row['total'];

37. How do you update data in a MySQL database using PHP?
    • Answer: Use the UPDATE SQL statement. Example:

<?php
$query = "UPDATE table SET column = 'value' WHERE condition";
mysqli_query($connection, $query);

38. How do you delete data from a MySQL database using PHP?
    • Answer: Use the DELETE SQL statement. Example:

<?php
$query = "DELETE FROM table WHERE condition";
mysqli_query($connection, $query);

39. What is a prepared statement and why is it used?
    • Answer: A prepared statement is used to execute the same or similar SQL statements repeatedly with high efficiency and to prevent SQL injection. They separate SQL logic from data values.
40. How do you use LIMIT in SQL with PHP?
    • Answer: Use the LIMIT clause to restrict the number of rows returned. Example:

<?php
$query = "SELECT * FROM table LIMIT 10";
$result = mysqli_query($connection, $query);

Object-Oriented Programming (OOP) Questions

41. What is object-oriented programming in PHP?
    • Answer: Object-oriented programming (OOP) is a programming paradigm based on objects, which are instances of classes. It focuses on using objects to design and build applications.
42. How do you define a class in PHP?
    • Answer: Use the class keyword followed by the class name and a pair of curly braces. Example:

<?php
class MyClass {
    // Properties and methods
}

43. What are properties and methods in a PHP class?
    • Answer: Properties are variables within a class, and methods are functions defined within a class.
44. What is inheritance in PHP?
    • Answer: Inheritance is a feature of OOP that allows a class to inherit properties and methods from another class using the extends keyword.
45. What is polymorphism in PHP?
    • Answer: Polymorphism allows objects of different classes to be treated as objects of a common superclass. It is often achieved through method overriding and interfaces.
46. What is encapsulation in PHP?
    • Answer: Encapsulation is the concept of wrapping data (properties) and code (methods) together as a single unit and restricting access to some of the object’s components.
47. How do you create an object in PHP?
    • Answer: Use the new keyword followed by the class name. Example:

<?php
$object = new MyClass();

48. What is an interface in PHP?
    • Answer: An interface is a contract that defines the methods a class must implement. It can only contain method signatures and constants. Example:

<?php
interface MyInterface {
    public function myMethod();
}

49. How do you implement an interface in PHP?
    • Answer: Use the implements keyword in the class definition. Example:

<?php
class MyClass implements MyInterface {
    public function myMethod() {
        // Implementation
    }
}

50. What is method overloading in PHP?
    • Answer: PHP does not support traditional method overloading like other languages, but you can use magic methods like __call() to simulate it.

Error Handling and Debugging Questions

51. How do you handle errors in PHP?
    • Answer: Errors can be handled using custom error handlers set by set_error_handler(), and exceptions can be handled using try-catch blocks.
52. What is an exception in PHP?
    • Answer: An exception is an object that describes an error or unexpected behavior. It can be thrown using throw and caught using try-catch blocks.
53. How do you throw an exception in PHP?
    • Answer: Use the throw keyword followed by an instance of the Exception class. Example:

<?php
if ($condition) {
    throw new Exception('Error message');
}

54. How do you catch an exception in PHP?
    • Answer: Use a try-catch block. Example:

<?php
try {
    // Code that may throw an exception
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
}

55. What is the purpose of finally in a try-catch block?
    • Answer: The finally block contains code that is always executed after the try and catch blocks, regardless of whether an exception was thrown.
56. What are PHP error reporting levels?
    • Answer: Error reporting levels include E_ERROR, E_WARNING, E_PARSE, E_NOTICE, E_CORE_ERROR, E_CORE_WARNING, E_COMPILE_ERROR, E_COMPILE_WARNING, E_USER_ERROR, E_USER_WARNING, E_USER_NOTICE, E_STRICT, and E_ALL.
57. How do you change PHP error reporting level?
    • Answer: Use the error_reporting() function or set the error_reporting directive in php.ini.

<?php
error_reporting(E_ALL);

58. What is the difference between include and require in PHP?
    • Answer: Both include and evaluate the specified file, but require will cause a fatal error if the file is not found, while include will only produce a warning and the script will continue.
59. How do you log errors in PHP?
    • Answer: Use the error_log() function to send an error message to the web server’s error log, a file, or a custom error handler.
60. What are the different types of errors in PHP?
    • Answer: The main types of errors in PHP are parse errors, fatal errors, warning errors, and notice errors.

Performance and Optimization Questions

61. How do you optimize PHP scripts for performance?
    • Answer: Techniques include using proper indexing in databases, reducing the use of nested loops, minimizing the use of global variables, caching, using efficient algorithms, and optimizing database queries.
62. What is opcode caching in PHP?
    • Answer: Opcode caching stores precompiled bytecode of PHP scripts to avoid parsing and compiling the code on each request. Examples include APC, XCache, and Zend OPcache.
63. How do you use caching in PHP?
    • Answer: Caching can be implemented using various methods such as file caching, memory caching (using tools like Memcached or Redis), and opcode caching.
64. What is output buffering in PHP?
    • Answer: Output buffering allows you to control when output is sent to the browser. It can be started with ob_start() and ended with ob_end_flush().
65. How do you measure the performance of a PHP script?
    • Answer: Performance can be measured using profiling tools like Xdebug, Blackfire, and New Relic, or using built-in functions like microtime() to measure script execution time.
66. What is the purpose of the header() function in PHP?
    • Answer: The header() function sends a raw HTTP header to the client. It is used to redirect pages, set content types, and control cache behavior.
67. How do you prevent SQL injection in PHP?
    • Answer: Use prepared statements and parameterized queries with mysqli or PDO.
68. How do you manage session data in PHP?
    • Answer: Session data can be managed using $_SESSION superglobal, session_start(), session_destroy(), and other session functions.
69. What is the difference between session_start() and session_regenerate_id()?
    • Answer: session_start() starts a new session or resumes an existing one, while session_regenerate_id() generates a new session ID to prevent session fixation attacks.
70. How do you handle file uploads in PHP?
    • Answer: File uploads are handled using the $_FILES superglobal, move_uploaded_file() function, and ensuring proper validation and sanitization of uploaded files.

Miscellaneous Questions

71. What is Composer in PHP?
    • Answer: Composer is a dependency manager for PHP that allows you to manage libraries and packages for your project.
72. How do you define and use namespaces in PHP?
    • Answer: Namespaces are defined using the namespace keyword. They help in avoiding name collisions between classes, functions, and constants.

<?php
namespace MyNamespace;
class MyClass {
    // Code
}

73. What is a RESTful API and how do you implement it in PHP?
    • Answer: A RESTful API is an architectural style for designing networked applications. It uses HTTP requests to perform CRUD operations. It can be implemented using frameworks like Laravel or Slim.
74. How do you send an email in PHP?
    • Answer: Use the mail() function or libraries like PHPMailer to send emails.
75. What is the difference between unlink() and unset() in PHP?
    • Answer: unlink() deletes a file from the file system, while unset() destroys a variable.
76. How do you handle JSON data in PHP?
    • Answer: Use json_encode() to encode data to JSON format and json_decode() to decode JSON data.
77. What is the purpose of the __construct() method in PHP?
    • Answer: The __construct() method is a constructor method that is automatically called when an object of the class is created.
78. What is the purpose of the __destruct() method in PHP?
    • Answer: The __destruct() method is a destructor method that is automatically called when an object is destroyed.
79. How do you create a constant in PHP?
    • Answer: Use the define() function. Example: define('CONSTANT_NAME', 'value');.
80. What is the difference between array_merge() and array_combine() in PHP?
    • Answer: array_merge() merges the elements of one or more arrays, while array_combine() creates an array by using one array for keys and another for values.

Security Questions

81. How do you secure user input in PHP?
    • Answer: Validate and sanitize all user inputs using functions like filter_var(), htmlspecialchars(), and regular expressions.
82. What is cross-site scripting (XSS) and how do you prevent it in PHP?
    • Answer: XSS is an attack where malicious scripts are injected into trusted websites. Prevent it by escaping output using functions like htmlspecialchars() and htmlentities().
83. What is cross-site request forgery (CSRF) and how do you prevent it in PHP?
    • Answer: CSRF is an attack that tricks the victim into submitting a malicious request. Prevent it by using anti-CSRF tokens and verifying them with each request.
84. What is the purpose of strip_tags() function in PHP?
    • Answer: The strip_tags() function removes HTML and PHP tags from a string.
85. How do you hash passwords in PHP?
    • Answer: Use the password_hash() function to create a secure hash of a password and password_verify() to verify it.
86. What is the difference between password_hash() and md5() in PHP?
    • Answer: password_hash() creates a secure hash with a built-in salt and is recommended for password hashing, while md5() creates an MD5 hash which is not secure for password storage.
87. What are prepared statements and why are they important?
    • Answer: Prepared statements are used to execute SQL queries securely by separating the query structure from data values, preventing SQL injection attacks.
88. How do you prevent session hijacking in PHP?
    • Answer: Use secure cookies, regenerate session IDs, validate user agents and IP addresses, and use HTTPS.
89. What is the purpose of session_regenerate_id() in PHP?
    • Answer: session_regenerate_id() generates a new session ID to prevent session fixation attacks.
90. How do you protect against file upload vulnerabilities in PHP?
    • Answer: Validate and sanitize file names, restrict file types and sizes, store uploaded files outside the web root, and use secure file handling functions.

Frameworks and Tools Questions

91. What is Laravel and why is it used?
    • Answer: Laravel is a PHP framework designed for building web applications using an expressive and elegant syntax. It provides tools for routing, authentication, sessions, and more.
92. What is Symfony in PHP?
    • Answer: Symfony is a PHP framework and a set of reusable PHP components for building web applications and APIs.
93. How do you install Composer in PHP?
    Answer: Download and install Composer using the command php -r copy('https://getcomposer.org/ installer', 'composer-setup.php');
    followed by php composer-setup.php.
94. What is CodeIgniter in PHP?
    • Answer: CodeIgniter is a lightweight PHP framework for building dynamic web applications. It is known for its small footprint and ease of use.
95. What is PHPUnit in PHP?
    • Answer: PHPUnit is a unit testing framework for PHP, used to write and run tests to ensure that code works as expected.
96. How do you use the Blade template engine in Laravel?
    • Answer: Blade is Laravel’s templating engine. Create a .blade.php file and use Blade syntax such as @if, @foreach, and @include for templating.
97. What is the difference between Laravel and Lumen?
    • Answer: Laravel is a full-featured web application framework, while Lumen is a micro-framework derived from Laravel, designed for building microservices and APIs.
98. How do you use Eloquent ORM in Laravel?
    Answer: Eloquent is Laravel’s ORM for working with databases. Define models that extend Illuminate\Database\Eloquent\Model
    and use Eloquent methods for database operations.
99. What is a middleware in Laravel?
    • Answer: Middleware is a mechanism for filtering HTTP requests entering your application. It provides a convenient way to inspect and filter requests.
100. What are service providers in Laravel? – Answer: Service providers are the central place of all Laravel application bootstrapping. They bind services into the service container and are registered in the config/app.php file.

These questions cover a wide range of PHP topics and are designed to test both fundamental and advanced knowledge of PHP programming. Good luck with your interview preparation!