The Top 8 PHP Interview Questions Every Hiring Manager Should Ask

php interview questions


The world of software development evolves rapidly. According to W3Techs, PHP is used by 76.8% of all the websites with a known server-side programming language, highlighting its profound influence on the internet’s infrastructure. 

This great usage of PHP in various domains creates opportunities, leading to a growing demand for proficient PHP developers. However, with high demand comes high standards. 

According to Devjobscanner, in 2023, PHP accounts for 10% of the total demand for developer job offers. Understanding the importance of PHP becomes valuable for developers seeking to showcase their PHP prowess and for recruiters intent on hiring the best talent.

In this article, we will explore the top 8 PHP interview questions crucial for hiring managers to assess developer proficiency. We will also provide code snippets to aid hiring managers in discerning the depth of a candidate’s understanding during PHP interviews.

What is PHP?

PHP stands for Hypertext Preprocessor. It’s a widely adopted, open-source scripting language. While its primary design was for web development, it also serves as a general-purpose programming language. The main application of PHP scripts is on the server side, implying that they execute on the web server. This capability positions PHP as a formidable tool for crafting dynamic and interactive websites and web applications, a reason for its widespread presence on the internet.

What sets PHP apart is its adaptability. Developers have the advantage of embedding PHP directly into HTML, bypassing the need to invoke an external file for data processing. Moreover, PHP boasts smooth integration capabilities with various databases, including MySQL, Oracle, and Microsoft SQL Server. Its compatibility extends to a plethora of web servers, and it operates efficiently on diverse platforms, be it Windows, Linux, Unix, or Mac OS X. This flexibility gifts developers with an expansive toolkit.

Popular websites that use PHP:

  1. Facebook
  2. Wikipedia
  3. Tumblr
  4. Slack
  5. MailChimp
  6. Etsy
  7. WordPress

These websites have millions of users and handle billions of requests per day.

Considering its strength in server-side scripting, PHP often finds use in tasks such as collecting form data, managing files on the server, tracking sessions, and even orchestrating comprehensive e-commerce platforms. This multifaceted utility underscores the reason PHP expertise is in demand across many tech-focused positions.

Top 8 PHP Interview Questions

The complexity and depth of PHP interview questions can range widely based on the specific role and the level of expertise needed. In this section, we will explore 8 PHP interview questions that serve as an important resource for developers seeking to sharpen their skills or for hiring teams aiming to gauge candidate abilities accurately.

1. Array Manipulation

Task: Write a PHP function named findMissingNumber that receives an array of consecutive numbers and identifies the missing number.
Input Format: An array of integers.
  1. The provided array will consist of consecutive integers with a single missing integer.
  2. A minimum of two elements will be present in the array.
Output Format: An integer representing the missing number.
Sample Input: [1, 2, 4, 5, 6]
Suggested Answer:
function findMissingNumber($numbers) {

$count = count($numbers);

$total = (($count + 1) * ($count + 2)) / 2;

$sum = array_sum($numbers);

return $total – $sum;


print_r(findMissingNumber(array(1, 2, 4, 5, 6)));


Output: 3
Why 3 is the answer? The solution uses a mathematical formula to calculate the expected sum of consecutive numbers and subtracts the actual sum of the given array from it. The result is the missing number.
Common mistakes to watch out for:


  1. Not taking into account that only one number is missing.
  2. Using iterative methods, which may not be efficient for larger arrays.


Can you optimize the function for larger arrays?

How would you handle negative numbers?

What the question tests?


The main goal of this question is to test the candidate’s ability to use built-in PHP functions for array manipulation and mathematical computations.

2. Handling Exceptions and Working with Files

Task: Write a PHP function named readFileAndSumNumbers, which reads a file containing numbers (each number on a new line). The function should parse these numbers and return their sum.
Input Format: A string representing the file path.
  1. The file will have at least one number.
  2. The file can have empty lines or lines with characters that aren’t numbers.
  3. Every number within the file will be an integer.
Output Format: An integer representing the combined sum of all the numbers in the file. Lines that can’t be converted to numbers should be disregarded.
Sample Input: “numbers.txt” (file contents: 1\n2\n3\nfoo\n4\n5\nbar\n)
Suggested Answer:
function readFileAndSumNumbers($filePath) {

    $sum = 0; 

    try {

        $file = new SplFileObject($filePath);       

        while (!$file->eof()) {

            $line = $file->fgets();           

            if (is_numeric($line)) {

                $sum += (int)$line;



    } catch (Exception $e) {

        echo “An error occurred: “ . $e->getMessage();


    return $sum;


echo readFileAndSumNumbers(“numbers.txt”);


Output: 15
Why 15 is the answer? The provided solution adopts the SplFileObject class, which provides an interface to access and manipulate file-related operations. It reads each line from the file, checks if it’s numeric, and then adds to the sum if true.
Common Mistakes to Watch Out For:
  1. Not handling exceptions properly, which can lead to system crashes if the file doesn’t exist or isn’t accessible.
  2. Not checking for numeric values correctly, potentially causing non-numeric values to be added or throwing errors.
  1. How would you modify the function to return the average of the numbers instead of the sum?
  2. How can you ensure the file is properly closed after reading?
What the Question tests? The main objective of this question is to gauge how well a developer can blend various skills: file handling, type checking, exception handling, and basic arithmetic operations in PHP.

3. Regular Expressions

Task: Write a PHP function by the name isValidEmail. This function should accept a string and employ a regular expression to determine its validity as an email address.
Input Format: The input should be in string format.
Constraints: The string must have a minimum length of one character.
Output Format: Output should be boolean. The function should return true for valid email addresses and false for invalid ones.
Sample Input:
Suggested Answer:
function isValidEmail($email) {

    return (filter_var($email, FILTER_VALIDATE_EMAIL)) ? true : false;


echo isValidEmail(‘’) ? ‘Valid’ : ‘Invalid’;

Output: true
Why True is the answer? The solution uses PHP’s filter_var() function combined with the FILTER_VALIDATE_EMAIL filter. This ensures that the provided string adheres to the criteria of a valid email address.
Common Mistakes to Watch Out For:
  1. Overcomplicating the solution by manually crafting a regular expression instead of leveraging built-in PHP functions.
  2. Not accounting for edge cases in email validation, such as domain extensions or special characters.
  1. How would you modify the function to validate that the email’s domain exists?
  2. Are there any limitations to using filter_var() with FILTER_VALIDATE_EMAIL in certain scenarios?
What the Question tests? The goal of this question is to test a candidate’s understanding of PHP’s built-in functions and ability to handle string validation tasks that are common in real-world applications.

 4. Object-Oriented Programming

  1. Create a PHP class named Circle. This class should encompass a private property called radius.
  2. Add a constructor for defining the radius.
  3. Integrate two public methods: getArea and getCircumference.

These methods will be employed to compute the area and the circumference of the circle in that order.

Suggested Answer:
class Circle {

    private $radius;

    public function __construct($radius) {

        $this->radius = $radius;


    public function getArea() {

        return pi() * pow($this->radius, 2);


    public function getCircumference() {

        return 2 * pi() * $this->radius;



$circle = new Circle(5);

echo “Area: “ . $circle->getArea();

echo “\nCircumference: “ . $circle->getCircumference();

Code Explanation The solution represents a typical representation of a circle using object-oriented programming in PHP. It encapsulates the circle’s properties and related methods inside a class, ensuring a clear and organized structure.
Common Mistakes to Watch Out For:
  1. Not using proper encapsulation by making the radius property public instead of private.
  2. Incorrect formulas for area or circumference.
  3. Failing to utilize built-in PHP functions like pi() and pow() for mathematical computations.
  1. Why is the radius property declared private, and why might that be important?
  2. How could you extend this class to incorporate other shapes or functionalities, like resizing the circle?
What the Question tests? The primary goal of this question is to gauge a candidate’s comprehension of the fundamentals of object-oriented programming in PHP. This includes an understanding of concepts like classes, properties, methods, and constructors.

5. Dealing with Databases

Task: Design a PHP function named fetchUsers. The primary role of this function is to establish a connection to a MySQL database. Once connected, it should retrieve all the entries from the ‘users’ table and subsequently return these records as an array.
Suggested Answer:
function fetchUsers($hostname, $username, $password, $dbname) {

    // Create connection

    $conn = new mysqli($hostname, $username, $password, $dbname);  

    // Check connection

    if ($conn->connect_error) {

        die(“Connection failed: “ . $conn->connect_error);


    $sql = “SELECT * FROM users”;

    $result = $conn->query($sql);

    $users = [];

    while($row = $result->fetch_assoc()) {

        $users[] = $row;



    return $users;



Code Explanation The solution establishes a database connection using the MySQLi extension in PHP. After ensuring the connection is successful, it executes an SQL query to fetch all records from the ‘users’ table. The results are then stored in an array, which is returned by the function.
Common Mistakes to Watch Out For:
  1. Not handling database connection errors, which could lead to undescriptive failure messages or application crashes.
  2. Forgetting to close the database connection after operations are completed.
  3.  Not validating or sanitizing inputs, potentially leading to SQL injection vulnerabilities.
  1. How would you modify this function to handle other types of databases, such as PostgreSQL?
  2. How would you incorporate prepared statements to enhance security against SQL injection?
  3. Discuss how you’d handle more complex queries or join operations to fetch data from multiple tables.
What the Question tests? This question evaluates a candidate’s proficiency with database connectivity in PHP, explicitly using the MySQLi extension. It tests their understanding of basic SQL queries and handling of database connection and retrieval operations.
6. Design Patterns
Task: Craft an implementation of the Singleton design pattern using PHP.
Suggested Answer:
class Singleton {

    // Hold the class instance.

    private static $instance = null;

    // The constructor is private to prevent initiation with outer code.

    private function __construct() {}

    // The object is created from within the class itself only if the class has no instance.

    public static function getInstance() {

        if (self::$instance == null) {

            self::$instance = new Singleton();


        return self::$instance;



Code Explanation The Singleton design pattern ensures that a particular class has only one instance and provides a way to access its sole instance from any other code point. 

The provided solution follows this pattern:

  • Making the constructor private so the class cannot be instantiated from outside.
  • Utilizing a private static property $instance to hold the single instance of the class.
  • Introducing a public static method getInstance() that either returns the existing instance or creates a new one if none exists.
Common Mistakes to Watch Out For:
  1. Not making the constructor private, allowing external code to create multiple instances.
  2. Forgetting to check for existing instances before creating a new one, leading to potential multiple instances.
  3. Not making the $instance property static would make it tied to an instance rather than the class.
  1. Why is the Singleton pattern sometimes criticized? What are its potential downsides in software design?
  2. How does the Singleton pattern differ from other creational design patterns?
  3. Discuss scenarios where the Singleton pattern would be an appropriate choice and where it might not be suitable.
What the Question tests? The objective of this PHP question is to test a candidate’s understanding of design patterns in PHP, specifically the Singleton pattern. It also requires a good understanding of object-oriented programming principles in PHP, including classes, methods, variable visibility (public, private, protected), and static properties and methods.

7. Memory Management

Task: Write a PHP function named calculateMemoryUsage. This function should accept a large array as its parameter, conduct a specific action on the array (for example, sorting it), and subsequently determine and return the variance in memory consumption before and after executing the operation.
Suggested Answer:
function calculateMemoryUsage($largeArray) {

    $startMemory = memory_get_usage();


    $endMemory = memory_get_usage();

    $memoryUsed = $endMemory $startMemory;

    return $memoryUsed;


Code Explanation The provided function calculateMemoryUsage takes the following steps:

  1. Determines the current memory usage before performing any operations using memory_get_usage().
  2. Sorts the input array using PHP’s built-in sort() function.
  3. Determines the memory usage after the sorting operation.
  4. Calculates the difference between the initial and final memory usage, which gives the amount of memory used during the operation.
  5. Returns the calculated memory difference.
Common Mistakes to Watch Out For:
  1. Not recording memory usage both before and after the operation would lead to incorrect calculations.
  2. Modifying the original array in ways other than what’s described in the task causes memory measurements to be off.
  3. Using functions other than memory_get_usage() may not give accurate or relevant results.
  1. In what scenarios might PHP’s memory usage be a concern, and how can you mitigate excessive memory consumption?
  2. How does PHP’s garbage collector work, and how does it help in managing memory?
  3. What are other methods or tools you can use to monitor and optimize memory usage in PHP applications?
What the Question tests? This question aims to evaluate a candidate’s awareness and understanding of PHP’s memory management mechanisms. It gauges how well they can measure the impact of specific operations on memory usage and indicates their proficiency in optimizing PHP code, especially when handling large data sets.

8. HTTP and Session Management

Task: Write a PHP script that initiates a fresh session, defines a session variable, and subsequently extracts the value of that specific session variable.
Suggested Answer:
// Start the session


// Set session variable

$_SESSION[“favorite_color”] = “blue”;

// Get session variable

$favoriteColor = $_SESSION[“favorite_color”];

Code Explanation The solution involves the following steps:

  1. Initiating a new session using the session_start() function.
  2. Setting a session variable named “favorite_color” with a value of “blue“.
  3. Retrieving and storing the value of the session variable “favorite_color” in the $favoriteColor variable.
Common Mistakes to Watch Out For:
  • Forgetting to start the session using session_start() before using the $_SESSION superglobal.
  • Incorrectly spelling or referencing session variable names, leading to undefined index errors.
  • Not understanding the life cycle of a session or how data persists across different requests.
  1. How can you destroy a session in PHP, and why might you want to do so?
  2. What precautions should you take when storing sensitive data in session variables?
  3. Describe other mechanisms in PHP for managing state across web requests. How do cookies differ from sessions in PHP?
What the Question tests? This question assesses a candidate’s understanding of session management in PHP within the context of the HTTP protocol. It evaluates their ability to initiate, maintain, and retrieve data across different HTTP requests, an essential skill in building interactive web applications.

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Structured and effective PHP technical interviews are important in hiring the best talent for your organization. These interviews provide a reliable assessment of a candidate’s technical understanding, problem-solving capabilities, approach to coding challenges, and adaptability to real-world scenarios. An effective interview process can make a huge difference between hiring a merely competent developer and securing a top-tier professional who can drive innovation and growth.

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