Top 25+ PHP Interview Questions & Answers | Personalized AI Support

Crack the PHP Interview: 20 Must-Know Q&A to Get You Hired

Q1: What is PHP and what are its main uses?

PHP (Hypertext Preprocessor) is a widely-used open-source scripting language designed for web development. It is embedded within HTML and interacts with databases, making it a powerful tool for creating dynamic and interactive web pages. PHP is used for:

  • Server-side scripting
  • Command-line scripting
  • Writing desktop applications

Q2: How does PHP handle form data?

PHP handles form data using the $_GET and $_POST superglobals. The $_GET method collects data sent via URL parameters, while $_POST collects data sent via an HTTP POST request.

// Example of handling form data using POST
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $name = $_POST['name'];
    echo "Name: " . htmlspecialchars($name);
}

Q3: What are PHP sessions and how do you use them?

PHP sessions allow you to store user information on the server for later use (e.g., user preferences, login status). Sessions are started with session_start() and variables are stored in the $_SESSION superglobal.

// Starting a session and setting a session variable
session_start();
$_SESSION['username'] = 'JohnDoe';
echo "Session username is " . $_SESSION['username'];

Q4: Explain the difference between include and require in PHP.

Both include and require are used to include files in PHP, but they differ in their behavior when an error occurs:

  • include : Generates a warning if the file cannot be included, and the script continues execution.
  • require : Generates a fatal error if the file cannot be included, and the script stops execution.
// Example of using include and require
include 'header.php'; // If header.php is missing, a warning is shown
require 'config.php'; // If config.php is missing, a fatal error occurs

Q5: What are the differences between == and === operators in PHP?

The == operator checks for value equality, meaning it will return true if the values on either side are equal, regardless of their type. The === operator checks for both value and type equality, meaning it will return true only if the values and types on either side are identical.

$a = 5;
$b = '5';

var_dump($a == $b);  // true, because values are equal
var_dump($a === $b); // false, because types are different (int vs. string)

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Q6: What is the difference between stack memory and heap memory in C++?

Stack Memory:

  • Used for static memory allocation.
  • Managed automatically by the compiler.
  • Limited in size.
  • Fast access.
  • Memory is deallocated when the function call ends.

Heap Memory:

  • Used for dynamic memory allocation.
  • Managed manually by the programmer.
  • Larger in size compared to stack.
  • Slower access.
  • Memory must be explicitly deallocated using delete or free .

Q7: Explain how error handling works in PHP.

PHP has several methods for handling errors, including:

  • Error Reporting: Configuring which types of errors are reported.
  • Custom Error Handlers: Using set_error_handler() to define custom error-handling functions.
  • Exception Handling: Using try , catch , and throw for handling exceptions.
// Example of exception handling
try {
    throw new Exception("An error occurred");
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
}

Q8: What is a PHP Data Object (PDO)?

PDO is a database access layer providing a uniform method of access to multiple databases. It allows for prepared statements, which are safer and more efficient for executing SQL queries.

// Example of using PDO to connect to a database
try {
    $pdo = new PDO('mysql:host=localhost;dbname=test', 'root', '');
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $pdo->prepare('SELECT * FROM users WHERE id = :id');
    $stmt->execute(['id' => 1]);
    $user = $stmt->fetch();
    print_r($user);
} catch (PDOException $e) {
    echo 'Connection failed: ' . $e->getMessage();
}

Q9: What are traits in PHP?

Traits are a mechanism for code reuse in single inheritance languages like PHP. A trait is similar to a class but is intended to group functionality in a fine-grained and consistent way. Traits can be included in multiple classes.

// Example of using traits
trait HelloWorld {
    public function sayHello() {
        echo 'Hello, World!';
    }
}

class MyClass {
    use HelloWorld;
}

$obj = new MyClass();
$obj->sayHello(); // Outputs: Hello, World!

Q10: How does PHP handle file uploads?

PHP handles file uploads via a global associative array called $_FILES . This array contains information about the uploaded files, such as name, type, size, and temporary location.

// Example of handling file uploads
if ($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_FILES['fileToUpload'])) {
    $target_dir = "uploads/";
    $target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
    if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
        echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
    } else {
        echo "Sorry, there was an error uploading your file.";
    }
}

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Q11: What is the difference between include, include_once, require, and require_once?

include : Includes a file. If the file is not found, a warning is issued, but the script continues.

include_once : Includes a file, but only once. If the file has already been included, it will not be included again.

require : Includes a file. If the file is not found, a fatal error is issued, and the script stops.

require_once : Includes a file, but only once. If the file has already been included, it will not be included again. If the file is not found, a fatal error is issued.

// Example usage
include 'file.php';
include_once 'file.php';
require 'file.php';
require_once 'file.php';

Q12: What is the $_SERVER superglobal in PHP?

The $_SERVER superglobal is an array that contains information about headers, paths, and script locations. It is populated by the web server and provides information about the environment in which the PHP script is running.

// Example usage
echo $_SERVER['PHP_SELF']; // Returns the filename of the currently executing script
echo $_SERVER['SERVER_NAME']; // Returns the name of the server host
echo $_SERVER['HTTP_USER_AGENT']; // Returns the user agent string
echo $_SERVER['SCRIPT_NAME']; // Returns the path of the current script

Q13: What is the purpose of the final keyword in PHP?

The final keyword in PHP can be used to prevent class inheritance or to prevent method overriding in derived classes. When a class is declared as final , it cannot be extended. Similarly, when a method is declared as final , it cannot be overridden by child classes.

// Example usage
final class BaseClass {
    final public function display() {
        echo "This is a final method.";
    }
}

// This will cause an error
class ChildClass extends BaseClass {
    // Cannot override final method
    public function display() {
        echo "Attempting to override.";
    }
}

Q14: Explain the use of the const keyword in PHP.

The const keyword is used to define constants in PHP. Constants are like variables except that once they are defined, they cannot be changed or undefined. Constants are defined using the const keyword inside a class or globally using the define() function.

// Example of defining a constant inside a class
class MyClass {
    const MY_CONSTANT = 'Constant Value';
    public function displayConstant() {
        echo self::MY_CONSTANT;
    }
}

$obj = new MyClass();
$obj->displayConstant(); // Outputs: Constant Value

// Example of defining a global constant
define('GLOBAL_CONSTANT', 'Global Value');
echo GLOBAL_CONSTANT; // Outputs: Global Value

Q15: What are anonymous functions in PHP?

Anonymous functions, also known as closures, are functions without a name. They are often used as callback functions or in situations where you need a one-time use function. Anonymous functions can capture variables from the surrounding scope using the use keyword.

// Example of an anonymous function
$greet = function($name) {
    echo "Hello, $name!";
};

$greet('John'); // Outputs: Hello, John!

// Example of an anonymous function capturing variables
$message = 'Goodbye';
$farewell = function($name) use ($message) {
    echo "$message, $name!";
};

$farewell('John'); // Outputs: Goodbye, John!

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Q16: What are PHP arrays and what types of arrays are there?

PHP arrays are data structures that can hold multiple values under a single name. They can store different types of values and can be accessed using indices or keys. There are three types of arrays in PHP:

  • Indexed arrays: Arrays with a numeric index.
  • $fruits = array("Apple", "Banana", "Orange");
    echo $fruits[0]; // Outputs: Apple
  • Associative arrays: Arrays with named keys.
  • $ages = array("John" => 25, "Jane" => 30);
    echo $ages["John"]; // Outputs: 25
  • Multidimensional arrays: Arrays containing one or more arrays.
  • $contacts = array(
        array("name" => "John", "email" => "john@example.com"),
        array("name" => "Jane", "email" => "jane@example.com")
    );
    echo $contacts[0]["name"]; // Outputs: John

Q17: What is the PDO::prepare() method and why is it important?

The PDO::prepare() method is used to prepare an SQL statement for execution. It is important because it allows the use of prepared statements, which enhance security by preventing SQL injection attacks. Prepared statements also optimize performance by allowing the database to reuse the same statement with different parameters.

// Example of using PDO::prepare()
$pdo = new PDO('mysql:host=localhost;dbname=test', 'root', '');
$stmt = $pdo->prepare('SELECT * FROM users WHERE email = :email');
$stmt->execute(['email' => 'john@example.com']);
$user = $stmt->fetch();
print_r($user);

Q18: How do you handle exceptions in PHP?

Exceptions in PHP are handled using the try , catch , and throw keywords. When an exception is thrown, the try block is exited, and the catch block is executed. This allows you to handle errors gracefully.

// Example of handling exceptions
try {
    if (!file_exists("somefile.txt")) {
        throw new Exception("File not found.");
    }
} catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "\n";
}

Q19: What is the purpose of the use keyword in PHP?

The use keyword in PHP has two main purposes:

  • To include traits within a class.
  • To import classes, functions, or constants from other namespaces.
// Example of using the use keyword to include a trait
trait Hello {
    public function sayHello() {
        echo "Hello!";
    }
}

class MyClass {
    use Hello;
}

// Example of using the use keyword for namespaces
namespace MyProject;

use SomeLibrary\SomeClass;

$obj = new SomeClass();

Q20: Explain the differences between echo and print in PHP.

Both echo and print are used to output data to the screen in PHP. However, there are some differences between them:

  • echo : Can take multiple parameters (although rarely used). It is slightly faster than print and does not return a value.
  • echo "Hello, ", "world!"; // Outputs: Hello, world!
  • print : Can only take a single argument and always returns 1, so it can be used in expressions.
  • if (print "Hello, world!") {
        // This block will always execute because print returns 1
    }

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Test Your PHP Skills: 10 Practical Assessment Based Q&A

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Q1: The following code is intended to print the numbers from 1 to 10, but it has some errors. Identify and fix the errors.
(Basic)

for ($i = 1; $i < 10; $i++) {
    echo $i;
}

The condition in the for loop should be $i <= 10 to include the number 10.


for ($i = 1; $i <= 10; $i++) {
    echo $i;
}
Q2: Predict the output of the following code snippet.
(Basic)

$a = 5;
$b = 10;
echo $a + $b;
$a = 20;
echo $a - $b;

The output will be 1520 because the concatenation of the results of the two echo statements without any space or separator.

// Output: 1520

Q3: The following code is supposed to concatenate two strings but it has a bug. Identify and fix the bug.
(Basic)

The variable $str3 is not defined. It should be $str2 .

$str1 = "Hello, ";
$str2 = "world!";
echo $str1 . $str2; // Outputs: Hello, world!
Q4: Predict the output of the following code snippet.
(Basic)

$array = array(1, 2, 3, 4);
foreach ($array as $value) {
    $value *= 2;
}
print_r($array);

The output will be the original array because $value is a copy of each element, not a reference.

// Output: Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 )

Q5: The following code calculates the sum of all even numbers from 1 to 100. Optimize it for better performance.
(Intermediate)

$sum = 0;
for ($i = 1; $i <= 100; $i++) {
    if ($i % 2 == 0) {
        $sum += $i;
    }
}
echo $sum;

We can optimize the code by directly iterating over even numbers.


$sum = 0;
for ($i = 2; $i <= 100; $i += 2) {
    $sum += $i;
}
echo $sum;
Q6: Write a PHP function that checks if a given string is a palindrome (reads the same backward as forward).
(Intermediate)

function isPalindrome($string) {
    $string = strtolower(preg_replace("/[^A-Za-z0-9]/", "", $string));
    return $string == strrev($string);
}

// Example usage:
echo isPalindrome("A man, a plan, a canal, Panama") ? "Palindrome" : "Not a palindrome"; 
// Outputs: Palindrome
Q7: The following code removes duplicate values from an array. Optimize it for better performance.
(Intermediate)

function removeDuplicates($array) {
    $result = array();
    foreach ($array as $value) {
        if (!in_array($value, $result)) {
            $result[] = $value;
        }
    }
    return $result;
}
$array = array(1, 2, 2, 3, 4, 4, 5);
print_r(removeDuplicates($array));

We can use the array_unique() function to improve performance.


function removeDuplicates($array) {
    return array_unique($array);
}
$array = array(1, 2, 2, 3, 4, 4, 5);
print_r(removeDuplicates($array)); // Outputs: Array ( [0] => 1 [1] => 2 [3] => 3 [4] => 4 [6] => 5 )
Q8: Write a PHP function that finds and returns the largest element in an array.
(Intermediate)

function findLargest($array) {
    if (empty($array)) {
        return null;
    }
    $max = $array[0];
    foreach ($array as $value) {
        if ($value > $max) {
            $max = $value;
        }
    }
    return $max;
}

// Example usage:
$array = array(1, 3, 7, 2, 5);
echo findLargest($array); // Outputs: 7
Q9: The following code is intended to calculate the factorial of a number but has errors and can be optimized. Identify the errors and optimize the code.
(Advanced)

function factorial($n) {
    $result = 1;
    for ($i = 1; $i <= $n; $i++) {
        $result *= $i;
    }
    return $result;
}
echo factorial(5);

The given code is correct but can be optimized using recursion.


function factorial($n) {
    if ($n === 0) {
        return 1;
    }
    return $n * factorial($n - 1);
}
echo factorial(5); // Outputs: 120
Q10: The following code is intended to sort an array of integers in ascending order using bubble sort but has errors and can be optimized. Identify the errors and optimize the code.
(Advanced)

function bubbleSort($array) {
    $n = count($array);
    for ($i = 0; $i < $n-1; $i++) {
        for ($j = 0; $j < $n-$i-1; $j++) {
            if ($array[$j] > $array[$j+1]) {
                $temp = $array[$j];
                $array[$j] = $array[$j+1];
                $array[$j+1] = $temp;
            }
        }
    }
    return $array;
}
$array = array(64, 34, 25, 12, 22, 11, 90);
print_r(bubbleSort($array));

The given code is correct but can be optimized using PHP's built-in sort() function for better performance.


function bubbleSort($array) {
    sort($array);
    return $array;
}
$array = array(64, 34, 25, 12, 22, 11, 90);
print_r(bubbleSort($array)); // Outputs: Array ( [0] => 11 [1] => 12 [2] => 22 [3] => 25 [4] => 34 [5] => 64 [6] => 90 )

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Overview of PHP

What is PHP?

What is the history and latest trends in PHP development?

What are some of the popular frameworks and libraries associated with PHP?

  • Laravel: A widely used PHP framework known for its elegant syntax and powerful features for web application development.
  • Symfony: A robust PHP framework for building complex web applications.
  • CodeIgniter: A lightweight PHP framework known for its simplicity and ease of use.
  • Zend Framework: An open-source framework for developing web applications and services using PHP.
  • Phalcon: A high-performance PHP framework delivered as a C extension.

What are the use cases of PHP?

  • Web Development: Building dynamic and interactive websites and web applications.
  • Content Management Systems (CMS): Developing CMS platforms like WordPress, Joomla, and Drupal.
  • E-commerce Platforms: Creating online stores and e-commerce solutions.
  • APIs: Developing RESTful APIs for web and mobile applications.
  • Server-Side Scripting: Handling server-side logic and database interactions.

What are some of the tech roles associated with expertise in PHP?

  • PHP Developer: Specializes in developing applications using PHP.
  • Web Developer: Focuses on both front-end and back-end development using PHP.
  • Full-Stack Developer: Utilizes PHP for back-end development along with front-end technologies.
  • CMS Developer: Develops and maintains content management systems using PHP.
  • API Developer: Creates and maintains APIs using PHP.

What pay package can be expected with experience in PHP?


Source: salary.com as of 2024

  • Junior PHP Developer: Typically earns between $60,000 and $80,000 per year.
  • Mid-Level PHP Developer: Generally earns from $80,000 to $100,000 per year.
  • Senior PHP Developer: Often earns between $100,000 and $120,000 per year.
  • Full-Stack Developer with PHP expertise: Generally earns between $85,000 and $110,000 per year.
  • Web Developer with PHP expertise: Typically earns between $75,000 and $95,000 per year.