8 PHP Interview Questions Every Developer Should Know

The world of software development evolves at a dizzying pace. Yet it is the oldest programming languages that continue to make the biggest impact. One such language that has consistently held its ground is PHP. Created in 1994, it’s the backbone of 77.4 percent of all websites with a known server-side programming language, highlighting its profound influence on the internet’s infrastructure. 

This widespread usage of PHP in various domains opens up a world of opportunities, leading to a growing demand for proficient PHP developers. However, with high demand comes high standards. Companies are on the lookout for professionals who can tackle complex problems, write efficient code, and effectively manage and manipulate data. Understanding the nuances of PHP, therefore, becomes a valuable skill, both for developers seeking to showcase their PHP prowess and for recruiters intent on hiring the best talent.

This post will dive into some PHP essentials and guide you through a series of progressively challenging PHP interview questions, providing illustrative code snippets and clear explanations to help you tackle your next PHP interview with confidence.

What is PHP?

PHP, an acronym for “Hypertext Preprocessor”, is a widely-used, open-source scripting language. It was specially designed for web development but is also used as a general-purpose programming language. PHP scripts are primarily used on the server side, meaning they run on the web server. It’s a robust platform for creating dynamic and interactive websites and web applications, which is part of what makes it so prevalent across the web.

The beauty of PHP lies in its flexibility. It can be embedded directly within HTML code, eliminating the need for calling an external file to process data. It also integrates seamlessly with various databases such as MySQL, Oracle, and Microsoft SQL Server. Plus, it’s compatible with most web servers and runs on various platforms (Windows, Linux, Unix, Mac OS X, etc.), providing developers with extensive versatility.

Given its server-side scripting prowess, PHP is typically used to perform operations like form data collection, file handling on the server, session tracking, or even building entire e-commerce websites. This wide range of applications explains why PHP skills are sought after for various technical roles.

What a PHP Interview Looks Like

The PHP interview process varies between organizations, but a few constants provide a broad picture of what to expect. These interviews are designed to assess a candidate’s understanding of PHP as a language, its applications, problem-solving skills, and sometimes their experience with PHP-based frameworks like Laravel or Symfony.

One of the first things to note is that PHP interviews often involve several layers. Typically, there’s an initial screening to evaluate the candidate’s basic PHP knowledge and possibly some related technologies like SQL or JavaScript. Following that, there’s usually a technical interview that delves deeper into PHP concepts and problem-solving skills.

A significant component of the technical interview is the coding challenge. These challenges test the ability of a candidate to apply their knowledge to solve real-world problems. They can range from simple tasks like string manipulation to more complex problems such as creating a simple CRUD (Create, Retrieve, Update, Delete) application or working with APIs.

The type of roles that require PHP skills vary widely but most commonly include web development roles. Web developers, particularly back-end developers, are often expected to have a solid understanding of PHP to create dynamic web pages or web applications. Other roles like software engineers might also need to know PHP, especially if they’re working on web-based applications. 

Even certain database administrator roles might require PHP knowledge, as PHP is commonly used to interact with databases. Additionally, since PHP can be used as a general-purpose scripting language, it could also come in handy for system administrators.

PHP Interview Questions

The complexity and depth of PHP interview questions can range widely, depending on the specific role and level of expertise required. This section will explore a progressive series of PHP coding challenges that span intermediate to advanced proficiency. These questions will serve as a vital resource for developers seeking to sharpen their skills or for hiring teams aiming to evaluate candidate competencies effectively.

1. Array Manipulation

Task: Write a PHP function called findMissingNumber that takes an array of consecutive numbers as input and returns the missing number.

Input Format: The input will be an array of integers.

Constraints:

• The array will contain consecutive integers, with one integer missing.
• The array will have at least two elements.

Output Format: The output will be an integer.

Sample Input: [1, 2, 4, 5, 6]

Sample Output: 3

Sample Code:

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)));

Explanation: 

The function first calculates the total sum of what the numbers would be if none were missing. Then it calculates the sum of the given numbers. The difference between these two sums is the missing number. This question tests 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 called readFileAndSumNumbers that reads a file with numbers (one number per line), parses the numbers, and returns their sum.

Input Format: The input will be a string representing the path to the file.

Constraints:

• The file will contain at least one number.
• The file may contain empty lines or lines with non-numeric characters.
• Each number will be an integer.

Output Format: The output will be an integer representing the sum of all numbers in the file. If a line cannot be parsed as a number, it should be ignored.

Sample Input: “numbers.txt” (file content: 1\n2\n3\nfoo\n4\n5\nbar\n)

Sample Output: 15

Sample Code:

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");

Explanation:

The readFileAndSumNumbers function starts by defining a sum variable to hold the total sum of the numbers. It then attempts to read each line from the file.

The is_numeric() function is used to check if each line can be treated as a number. If it can, the line is cast to an integer and added to the sum. If not, the line is ignored.

If an exception occurs during the execution of the code (such as a RuntimeException if the specified file doesn’t exist), the exception is caught and an error message is printed. Regardless, the function returns the sum of the parsed numbers.

This question tests a candidate’s ability to handle exceptions and work with files in PHP. It also provides an opportunity to demonstrate understanding of PHP’s type checking and casting mechanisms.

3. Regular Expressions

Task: Write a PHP function called isValidEmail that takes a string as input and uses a regular expression to verify if it is a valid email address. 

Input Format: The input will be a string.

Constraints: The string will contain at least one character.

Output Format: The output will be a boolean. Return true if the string is a valid email address and false otherwise.

Sample Input: test@example.com

Sample Output: true

Sample Code:

function isValidEmail($email) {
    return (filter_var($email, FILTER_VALIDATE_EMAIL)) ? true : false;
}
echo isValidEmail('test@example.com') ? 'Valid' : 'Invalid';

Explanation:

The isValidEmail function uses PHP’s built-in filter_var function with the FILTER_VALIDATE_EMAIL filter to validate the email address. If the email is valid, the function returns true; if it’s not, it returns false.

This question tests 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

Task: Define a PHP class called Circle, with a private property radius. Add a constructor to set the radius and two public methods: getArea and getCircumference to calculate the area and the circumference of the circle, respectively.

Sample Code:

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();

Explanation:

The Circle class is defined with a private property radius. The constructor method takes radius as a parameter and assigns it to the radius property. 

Two public methods, getArea and getCircumference, are defined to calculate the area and circumference of the circle. They use the pi() function to get the value of π, and pow() to calculate the square of the radius. 

This question tests a candidate’s understanding of object-oriented programming concepts in PHP, such as classes, properties, methods, and constructors.

5. Dealing with Databases

Task: Write a PHP function called fetchUsers that connects to a MySQL database, fetches all records from the users table, and returns the result as an array.

Sample Code:

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;
    }
   $conn->close();
    return $users;
}

Explanation:

The fetchUsers function first creates a new MySQLi object and establishes a connection with the MySQL database using the provided hostname, username, password, and database name. 

It then constructs a SQL query to fetch all records from the users table. The query is executed using the query() method of the MySQLi object. 

The function iterates over the result set using a while loop and the fetch_assoc() method, which fetches a result row as an associative array. Each row is added to the users array. 

Once all rows have been fetched, the database connection is closed using the close() method of the MySQLi object. 

This question tests the candidate’s understanding of PHP’s MySQLi extension, which allows PHP to access and manipulate MySQL databases, and their ability to interact with databases using SQL.

6. Design Patterns

Task: Implement a Singleton design pattern in PHP.

Sample Code:

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;
    }
}

Explanation:

The Singleton class is a design pattern that restricts a class from instantiating multiple objects. It’s useful when only a single instance of a class is required to control actions. 

It’s implemented by creating a class with a method that creates a new instance of the class if one doesn’t exist. If an instance already exists, it simply returns a reference to that object. To prevent additional instantiation, the constructor is made private.

In the provided sample code, Singleton is a class with a private static variable $instance to hold the single instance of the class, a private constructor to prevent external instantiation, and a public static getInstance method to control the access to the single instance of the class.

This question tests a candidate’s understanding of design patterns in PHP, specifically the Singleton pattern. It requires a good understanding of object-oriented programming principles in PHP, including classes, methods, and variables visibility (public, private, protected), and static properties and methods.

7. Memory Management

Task: Write a PHP function called calculateMemoryUsage that takes a large array as an input, performs a certain operation on the array (like sorting it), and then calculates and returns the difference in memory usage before and after the operation.

Sample Code:

function calculateMemoryUsage($largeArray) {
    $startMemory = memory_get_usage();
    sort($largeArray);
    $endMemory = memory_get_usage();
    $memoryUsed = $endMemory - $startMemory;
    return $memoryUsed;
}

Explanation:

The calculateMemoryUsage function uses PHP’s memory_get_usage function to get the amount of memory, in bytes, that’s currently being used by the PHP script. 

It captures the memory usage at the start, performs an operation (in this case, sorting the array), and then captures the memory usage again at the end. The difference between the end and start memory usages is calculated and returned, representing the additional memory used by the operation.

This question is a good opportunity for candidates to demonstrate an understanding of how PHP manages memory and the impact of different operations on memory usage. It’s an important consideration for writing efficient, performant PHP code, especially when dealing with large data sets.

8. HTTP and Session Management

Task: Write a PHP script to start a new session, set a session variable, and then retrieve the value of that session variable.

Sample Code:

// Start the session
session_start();
// Set session variable
$_SESSION["favorite_color"] = "blue";
// Get session variable
$favoriteColor = $_SESSION["favorite_color"];

Explanation:

In this task, the PHP session_start function is first called to start a new session or to resume the current one. This is necessary before any session variables can be set.

Then, a session variable “favorite_color” is set to “blue” using the global $_SESSION variable, which is an associative array containing session variables available to the current script.

Finally, the value of the “favorite_color” session variable is retrieved and stored in the $favoriteColor variable.

This question tests a candidate’s understanding of HTTP, sessions, and how PHP manages state across different web requests. In a stateless protocol like HTTP, sessions provide a way to store information about a user across multiple requests. Mastery of session management is key to developing interactive and personalized web applications with PHP.

Resources for Interview Preparation

• HackerRank Interview
• Tech Interview Prep: How To Ace Your Interview

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