PHP Interview Questions

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

Answer:

• == checks if values are equal, but not types.

• === checks if values are equal and of the same type.

Example:

$a = 5;       // integer
$b = '5';     // string

var_dump($a == $b);  // true
var_dump($a === $b); // false

Explanation:
$a == $b is true because both are 5.
But $a === $b is false because one is an integer and the other is a string.

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2. What are traits in PHP and why are they used?

Answer:
Traits are like reusable chunks of code. PHP doesn't support multiple inheritance (a class extending two classes), but traits let you use methods from multiple sources.

Example:

trait Logger {
    public function log($msg) {
        echo "Log: $msg";
    }
}

class MyClass {
    use Logger;
}

$obj = new MyClass();
$obj->log("Hello!"); 

Explanation:
Here, the Logger trait gives the log() method to MyClass, without inheritance. You can reuse traits across many classes.

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3. What is late static binding in PHP?

Answer:
Late static binding allows you to reference the called class, not the class where the method was originally defined.

Example:

class A {
    public static function who() {
        echo __CLASS__;
    }

    public static function test() {
        static::who(); // late static binding
    }
}

class B extends A {
    public static function who() {
        echo __CLASS__;
    }
}

B::test(); // Outputs: B

Explanation:
If we used self::who(), it would print A. But static::who() refers to the calling class (B), thanks to late static binding.

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4. What are generators in PHP?

Answer:
Generators are like lightweight iterators. They help loop over data without loading everything into memory.

Example:

function countUpTo($n) {
    for ($i = 1; $i <= $n; $i++) {
        yield $i;
    }
}

foreach (countUpTo(5) as $num) {
    echo $num . " ";
}

Explanation:
Instead of returning all values at once, yield returns one value at a time. It’s memory efficient for large data sets.

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5. What is the difference between public, protected, and private in PHP?

Answer:
These are access modifiers for class members (properties or methods):

• public – accessible from anywhere.

• protected – accessible only inside the class and its child classes.

• private – accessible only inside the class itself.

Example:

class Test {
    public $a = "Public";
    protected $b = "Protected";
    private $c = "Private";

    public function show() {
        echo $this->a . ", " . $this->b . ", " . $this->c;
    }
}

$obj = new Test();
$obj->show();     // Works
echo $obj->a;     // Works
// echo $obj->b;  // Error
// echo $obj->c;  // Error

Explanation:
You can access $a from outside. $b and $c are protected/private, so direct access gives an error.

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6. What is the use of final keyword in PHP?

Answer:
The final keyword prevents class or method from being overridden.

Example:

class A {
    final public function hello() {
        echo "Hello";
    }
}

class B extends A {
    // public function hello() {} // Error!
}

Explanation:
You can't override the hello() method in class B because it's marked final.

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7. What is the use of the __call() magic method in PHP?

Answer:
__call() is triggered when calling an undefined method in an object.

Example:

class Magic {
    public function __call($name, $arguments) {
        echo "Calling method '$name' with arguments: " . implode(', ', $arguments);
    }
}

$obj = new Magic();
$obj->sayHello("World");  // sayHello is not defined

Explanation:
Even though sayHello is not defined, __call() catches it and handles the call gracefully.

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8. How can you handle errors in PHP using try-catch?

Answer:
Use try-catch to handle exceptions and avoid fatal errors.

Example:

try {
    if (!file_exists("test.txt")) {
        throw new Exception("File not found!");
    }
} catch (Exception $e) {
    echo "Caught error: " . $e->getMessage();
}

Explanation:
If test.txt is missing, it throws an exception. The catch block handles it, preventing a crash.

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9. What is Dependency Injection in PHP?

Answer:
Dependency Injection is a design pattern where a class gets its dependencies from outside, not by creating them itself.

Example:

class Logger {
    public function log($msg) {
        echo $msg;
    }
}

class UserService {
    private $logger;

    public function __construct(Logger $logger) {
        $this->logger = $logger;
    }

    public function createUser() {
        $this->logger->log("User created!");
    }
}

$logger = new Logger();
$service = new UserService($logger);
$service->createUser();

Explanation:
UserService doesn’t create a Logger. It receives it via the constructor. This makes testing and managing dependencies easier.

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10. What is the difference between abstract class and interface in PHP?

Answer:

Abstract Class	Interface
Can have properties and methods	Can only have method signatures (until PHP 8)
Can have both defined and undefined methods	Only method declarations (no body, except static methods in PHP 8+)
Use abstract keyword	Use interface keyword
Supports inheritance	Supports multiple inheritance

Example:

interface Shape {
    public function getArea();
}

abstract class Polygon {
    public function sayHi() {
        echo "Hi from Polygon!";
    }
}

class Rectangle extends Polygon implements Shape {
    public function getArea() {
        return 10 * 20;
    }
}

Explanation:

• Interface is like a contract.

• Abstract class gives some shared functionality.

• You can implement multiple interfaces but only extend one abstract class.

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11. What is the difference between include, require, include_once, and require_once in PHP?
Answer:
These are used to include files in PHP, but they differ in error handling and repeat inclusion.

• include → throws a warning if the file is not found.

• require → throws a fatal error and stops execution if the file is not found.

• include_once / require_once → ensures the file is included only once.

Example:

include 'file.php';         // Warning if missing
require 'file.php';         // Fatal error if missing
include_once 'file.php';    // Included only once
require_once 'file.php';    // Included only once

Explanation:
Use require when the file is essential. Use include for optional files.

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12. What is the difference between static and non-static methods in PHP?
Answer:

• Static methods can be called without creating an object.

• Non-static methods require object instantiation.

Example:

class MyClass {
    public static function sayHi() {
        echo "Hi!";
    }

    public function sayHello() {
        echo "Hello!";
    }
}

MyClass::sayHi();              // Static method
$obj = new MyClass();
$obj->sayHello();              // Non-static method

Explanation:
Static methods are used for utility-style functions that don’t depend on object state.

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13. What is an anonymous function (closure) in PHP?
Answer:
An anonymous function is a function without a name, often used for short-lived operations or callbacks.

Example:

$greet = function($name) {
    return "Hello, $name!";
};

echo $greet("John");

Explanation:
Closures can be stored in variables, passed as parameters, and even use variables from outside using use.

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14. What is namespacing in PHP and why use it?
Answer:
Namespaces prevent class and function name collisions.

Example:

namespace App\Models;

class User {
    public function getName() {
        return "John";
    }
}

// Usage
$user = new \App\Models\User();

Explanation:
Namespaces are essential for organizing code in large applications.

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15. What is the SPL in PHP?
Answer:
SPL = Standard PHP Library. It provides a collection of interfaces and classes for common data structures (like LinkedList, Stack, etc).

Example:

$stack = new SplStack();
$stack->push("First");
$stack->push("Second");
echo $stack->pop(); // Second

Explanation:
SPL helps manage data structures without implementing them manually.

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16. How do you use type declarations in PHP?
Answer:
Type declarations enforce variable types for function parameters and return types.

Example:

function add(int $a, int $b): int {
    return $a + $b;
}

echo add(5, 3);  // Outputs: 8

Explanation:
This helps catch bugs early by ensuring the correct data types are used.

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17. What is object cloning in PHP?
Answer:
Cloning creates a shallow copy of an object using the clone keyword.

Example:

class MyClass {
    public $name = "Original";
}

$a = new MyClass();
$b = clone $a;
$b->name = "Cloned";

echo $a->name;  // Original
echo $b->name;  // Cloned

Explanation:
Each object becomes independent after cloning.

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18. What is the difference between isset(), empty(), and is_null()?
Answer:

• isset() → checks if variable is set and not null.

• empty() → checks if variable is empty.

• is_null() → checks if variable is null.

Example:

$var = 0;

var_dump(isset($var));    // true
var_dump(empty($var));    // true
var_dump(is_null($var));  // false

Explanation:
Use them according to what you're testing — existence, emptiness, or nullness.

examples, continuing in the same format:

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19. What is the use of the __get() and __set() magic methods in PHP?
Answer:
These magic methods are triggered when accessing or assigning values to inaccessible (private/protected/non-existent) properties.

Example:

class Demo {
    private $data = [];

    public function __get($name) {
        return $this->data[$name] ?? "Not found";
    }

    public function __set($name, $value) {
        $this->data[$name] = $value;
    }
}

$obj = new Demo();
$obj->name = "PHP";
echo $obj->name;  // Output: PHP

Explanation:
They allow dynamic handling of properties and can be used for proxies or ORM models.

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20. What is the difference between session and cookie in PHP?
Answer:

• Session stores data on the server.

• Cookie stores data in the user’s browser.

Example:

// Cookie
setcookie("user", "John", time() + 3600);

// Session
session_start();
$_SESSION["user"] = "John";

Explanation:
Sessions are more secure; cookies are suitable for persistent lightweight data.

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21. How does PHP handle references (&) and what is their use?
Answer:
References allow two variables to point to the same data.

Example:

$a = "Hello";
$b = &$a;
$b = "World";

echo $a;  // Output: World

Explanation:
Useful when you want one variable to reflect changes in another, like passing by reference in functions.

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22. What is the difference between unset() and null in PHP?
Answer:

• unset() destroys the variable.

• Setting a variable to null keeps it declared but with no value.

Example:

$a = "Hello";
unset($a);     // $a is no longer set

$b = "World";
$b = null;     // $b is still set, but null

Explanation:
Use unset() to remove variables completely.

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23. What is the use of the yield from expression in PHP?
Answer:
yield from is used to delegate part of a generator to another generator.

Example:

function gen1() {
    yield 1;
    yield 2;
}

function gen2() {
    yield from gen1();
    yield 3;
}

foreach (gen2() as $val) {
    echo $val . " ";  // Output: 1 2 3
}

Explanation:
Makes code cleaner by reducing nested loops in generators.

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24. What are weak references in PHP and when to use them?
Answer:
A weak reference allows referencing an object without preventing it from being garbage-collected.

Example:

$obj = new stdClass();
$weakRef = WeakReference::create($obj);

unset($obj);
var_dump($weakRef->get());  // NULL

Explanation:
Useful in caching systems where you don’t want to prevent object cleanup.

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25. What is the purpose of array_map() in PHP?
Answer:
array_map() applies a callback to each element in one or more arrays.

Example:

$numbers = [1, 2, 3];
$squares = array_map(fn($n) => $n * $n, $numbers);

print_r($squares);  // [1, 4, 9]

Explanation:
It’s a functional approach to manipulating arrays.

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26. How to create and use a custom exception in PHP?
Answer:
Extend the base Exception class to define your own exception type.

Example:

class MyException extends Exception {}

try {
    throw new MyException("Custom error!");
} catch (MyException $e) {
    echo $e->getMessage();  // Output: Custom error!
}

Explanation:
Custom exceptions help categorize and handle specific error types.

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27. What is the difference between explode() and str_split()?
Answer:

• explode() splits a string by a delimiter.

• str_split() splits a string into chunks (by length).

Example:

$str = "a,b,c";
print_r(explode(",", $str));   // ['a', 'b', 'c']

$str2 = "hello";
print_r(str_split($str2, 2));  // ['he', 'll', 'o']

Explanation:
Use based on whether you're splitting by characters or delimiters.

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28. What is the difference between foreach and each() in PHP?
Answer:

• foreach is a modern loop for arrays.

• each() was used to return the current key-value pair and move the pointer (deprecated in PHP 7.2+).

Example (foreach):

$arr = ["a" => 1, "b" => 2];
foreach ($arr as $key => $value) {
    echo "$key => $value ";
}

Explanation:
Avoid each() in modern code; prefer foreach for readability and compatibility.

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29. What is a recursive function in PHP?
Answer:
A function that calls itself to solve smaller sub-problems.

Example:

function factorial($n) {
    return $n <= 1 ? 1 : $n * factorial($n - 1);
}

echo factorial(5); // Output: 120

Explanation:
Used in problems like factorial, tree traversal, etc.

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30. What is the difference between array_merge() and array_merge_recursive()?
Answer:

• array_merge() overwrites values with the same key.

• array_merge_recursive() merges them into arrays.

Example:

$a = ['color' => 'red'];
$b = ['color' => 'blue'];

print_r(array_merge($a, $b));           // ['color' => 'blue']
print_r(array_merge_recursive($a, $b)); // ['color' => ['red', 'blue']]

Explanation:
Use recursive merge when you want to retain all values.

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31. How to serialize and unserialize data in PHP?
Answer:
Used to convert objects/arrays into a storable string format.

Example:

$data = ['name' => 'John'];
$serialized = serialize($data);
$unserialized = unserialize($serialized);

print_r($unserialized);

Explanation:
Used in sessions, caching, or storing complex data structures.

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31. What is the difference between array_filter() and array_map()?
Answer:

• array_filter() removes elements based on condition.

• array_map() modifies each element.

Example:

$nums = [1, 2, 3, 4];

$even = array_filter($nums, fn($n) => $n % 2 === 0); // [2, 4]
$squared = array_map(fn($n) => $n * $n, $nums);      // [1, 4, 9, 16]

Explanation:
Filter = keep only what matches. Map = change values.

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32. What is the difference between isset() and empty() in PHP?
Answer:

• isset() checks if a variable is set and not null.

• empty() checks if a variable is empty (0, '', null, false, etc.)

Example:

$a = 0;
var_dump(isset($a));  // true
var_dump(empty($a));  // true

Explanation:
isset() won’t return false for falsy values like 0, while empty() will.

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33. What is the Null Coalescing Operator (??) in PHP?
Answer:
It returns the first operand if it's set and not null; otherwise, returns the second.

Example:

$name = $_GET['name'] ?? 'Guest';
echo $name;

Explanation:
It’s a shorthand for isset($var) ? $var : default.

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34. How to prevent SQL Injection in PHP?
Answer:
Use prepared statements with parameter binding.

Example (PDO):

$pdo = new PDO($dsn, $user, $pass);
$stmt = $pdo->prepare("SELECT * FROM users WHERE email = ?");
$stmt->execute([$email]);

Explanation:
Prepared statements ensure user input is properly escaped, preventing injection.

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35. What is the difference between include, require, include_once, and require_once?
Answer:

• include gives a warning if the file is missing, script continues.

• require gives a fatal error, script stops.

• _once ensures the file is included only once.

Example:

include 'file.php';
require 'file.php';
include_once 'file.php';
require_once 'file.php';

Explanation:
Use require_once for essential files to avoid multiple inclusions.

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36. What is overloading and overriding in PHP OOP?
Answer:

• Overloading: Using magic methods (__get, __set, __call) to dynamically create properties/methods.

• Overriding: Redefining a parent class method in a child class.

Example (Overriding):

class A {
    public function greet() {
        echo "Hello";
    }
}
class B extends A {
    public function greet() {
        echo "Hi";
    }
}

$obj = new B();
$obj->greet();  // Hi

Explanation:
Overriding customizes base class behavior, while overloading adds dynamic flexibility.

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37. What are anonymous classes in PHP?
Answer:
Anonymous classes are one-off class definitions used when you don't need to reuse them.

Example:

$greeting = new class {
    public function say() {
        return "Hello from anonymous class!";
    }
};

echo $greeting->say();

Explanation:
Anonymous classes help when you need a simple class implementation without naming or reuse.

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38. What is the difference between explode() and str_split() in PHP?
Answer:

• explode() splits a string by a delimiter into an array.

• str_split() splits a string into individual characters or chunks of specified length.

Example:

$str = "Hello,World";
print_r(explode(",", $str));    // ['Hello', 'World']
print_r(str_split($str, 2));    // ['He', 'll', 'o,', 'Wo', 'rl', 'd']

Explanation:
Use explode() when breaking a string by a known separator. Use str_split() for character-level operations.

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39. What is array_walk() in PHP?
Answer:
array_walk() applies a user-defined function to every element of an array.

Example:

$arr = [1, 2, 3];
array_walk($arr, function (&$value, $key) {
    $value *= 2;
});

print_r($arr); // [2, 4, 6]

Explanation:
Useful for applying transformations with reference access to each value.

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40. What is a Closure in PHP?
Answer:
A Closure is an anonymous function that can capture variables from the parent scope.

Example:

$multiplier = 3;
$multiply = function ($n) use ($multiplier) {
    return $n * $multiplier;
};

echo $multiply(5); // 15

Explanation:
Closures are commonly used in callbacks, array functions, and functional programming.

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41. How does PHP handle timezones and date formatting?
Answer:
PHP uses the DateTime class and date_default_timezone_set() for timezones.

Example:

date_default_timezone_set('Asia/Kolkata');
$dt = new DateTime();
echo $dt->format('Y-m-d H:i:s');

Explanation:
Using DateTime gives you timezone-aware, formatted, and manipulable date/time values.

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42. What is the difference between unset() and null assignment in PHP?
Answer:

• unset() destroys the variable.

• Assigning null just empties its value.

Example:

$a = 5;
unset($a);  // Variable is removed

$b = 5;
$b = null;  // Variable still exists, value is null

Explanation:
isset($a) will return false after unset(), but true for a variable set to null.

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43. What are callable types in PHP?
Answer:
Callable types are functions/methods passed as variables, closures, or arrays.

Example:

function greet($name) {
    return "Hello, $name";
}

$call = 'greet';
echo $call('John'); // Hello, John

Explanation:
Useful for callbacks, array functions, and flexible APIs.

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44. How do you make a class iterable in PHP?
Answer:
Implement the Iterator or IteratorAggregate interface.

Example:

class MyList implements IteratorAggregate {
    private $items = [1, 2, 3];

    public function getIterator(): Traversable {
        return new ArrayIterator($this->items);
    }
}

$obj = new MyList();
foreach ($obj as $item) {
    echo $item . " ";
}

Explanation:
This allows your objects to be used in foreach just like arrays.

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45. What is __autoload() and how is it replaced in modern PHP?
Answer:

• __autoload() is an old method for loading undefined classes automatically.

• Replaced by spl_autoload_register() in modern PHP.

Example:

spl_autoload_register(function ($class) {
    include "classes/$class.php";
});

Explanation:
Modern autoloading allows registering multiple loaders and is compatible with Composer and PSR standards.

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46. How do you handle large file uploads in PHP?
Answer:

1. Update php.ini:

   • upload_max_filesize

   • post_max_size

   • max_execution_time

2. Use chunked uploads or background processing for huge files.

Explanation:
PHP has built-in limits for safety. For large files, it's common to handle chunks and optimize memory usage.

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47. What is the use of __toString() magic method in PHP?
Answer:
Allows an object to be treated as a string.

Example:

class Book {
    public $title = "PHP Guide";

    public function __toString() {
        return $this->title;
    }
}

$book = new Book();
echo $book; // Output: PHP Guide

Explanation:
When an object is used in a string context, __toString() is automatically called.

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