Top 55+ PHP Interview Questions

PHP is a scripting language specifically designed for Web Development. It is embedded within HTML code and executed on the server, generating dynamic web pages. PHP's popularity stems from several factors, including: 

a) Easy learning curve: PHP has a straightforward syntax similar to C and is relatively easy to grasp for beginners. 

b) Wide availability: PHP is supported by almost all web hosting providers and runs on major operating systems. 

c) Large community: PHP boasts a vast community of developers contributing to its extensive documentation, libraries, and frameworks. 

d) Flexibility: PHP can be used for various tasks, including server-side scripting, command-line scripting, and developing web applications. 

e) Integration: PHP can seamlessly integrate with various databases, such as MySQL, Oracle, and PostgreSQL.

The advantages of PHP are as follows:  

a) Open-source: PHP is an open-source language, meaning it is freely available, customisable, and regularly updated by the community.  

b) Platform independence: PHP can run on multiple platforms, including Windows, Linux, macOS, and various web servers.  

c) Extensive documentation: PHP has comprehensive documentation that makes learning and troubleshooting issues easy.  

d) Integration capabilities: PHP can integrate smoothly with databases, web services, and other technologies.  

e) Strong community support: PHP benefits from an active community that provides support, tutorials, and numerous libraries and frameworks.  

The disadvantages of PHP are as follows:  

a) Inconsistent standard library: PHP's standard library lacks consistency, leading to varying function names and parameter orders in different versions.  

b) Weak typing: PHP is a weakly typed language, which can lead to potential errors and make code harder to debug.  

c) Security concerns: If not used cautiously, PHP can fall prey to security risks such as SQL injections and Cross-site Scripting (XSS) attacks.  

d) Scalability challenges: PHP may face scalability issues when handling heavy loads or complex applications, requiring additional optimisation measures. 

HTML, a markup language, is used for structuring the content and layout of web pages. It provides a standardised way to define the elements that make up a web page, such as headings, paragraphs, links, images, and tables. HTML files are primarily static and can be rendered directly by web browsers.  

On the other hand, PHP is a server-side scripting language used to generate dynamic web content. It allows you to embed PHP code within HTML files, enabling the execution of server-side operations, such as database queries, form processing, and session management. PHP code is processed on the server before the resulting HTML is displayed on the client's browser. 

Variables in PHP are declared using the dollar symbol ($), followed by the variable name. The name should start with a letter or underscore, followed by letters, digits, or underscores. Here's an example of variable declaration and assignment:
 

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Superglobal variables in PHP are predefined variables that are accessible from anywhere in the PHP script without the need for special declarations. They provide valuable information and functionality for various purposes. Some commonly used superglobal variables in PHP include: 

a) $_GET: Contains the values passed through the URL parameters using the GET method. 

b) $_POST: Contains the values submitted through an HTML form using the POST method. 

c) $_SERVER: Provides server-related information, such as the current script's file path, host, and request headers. 

d) $_SESSION: Stores session data across multiple requests. 

e) $_COOKIE: Contains values stored in the user's browser as cookies. 

f) $_FILES: Contains information about uploaded files. 

GET and POST are HTTP methods that send data from a client (browser) to a server. Their main differences are as follows:  

a) GET method: Sends data as part of the URL parameters visible in the browser's address bar. It is suitable for non-sensitive data, such as search queries or publicly accessible pages. The data limit for GET requests is typically restricted.  

b) POST method: Sends data in the HTTP request body, making it hidden from the URL and more secure. It is suitable for sensitive or large data, such as login credentials or file uploads. The data limit for POST requests is usually larger than that of GET requests. 

The echo statement outputs text or variables. 

A PHP variable is a container for storing data values. 

The define() function is used to represent the Constants. 

The == operator checks for equality, while the === operator checks for equality and type. 

The strlen() function provides the length of a string. 

The include or require statement is used to include an external PHP file. 

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

Single-line comments in PHP start with //, and multi-line comments are enclosed between /* and */. 

Both print and echo output data, but echo is slightly faster and can handle multiple parameters. 

The count() function is used to get the length of an array. 

The count() function is used to get the length of an array. 

The array_unique() function removes duplicate values from an array. 

array_key_exists() checks if a specific key exists in an array, while isset() checks if a variable is set. 

The sort() function sorts an array in ascending order. 

The sort() function sorts an array in ascending order. 

This function converts a string to a lowercase. 

The date() function formats and displays the current date and time. 

This function sends a raw HTTP header to redirect users to another page. 

The $_GET variable retrieves values from a form submitted with the HTTP GET method. 

PHP uses a mix of HTML and PHP code to create dynamic web pages. The PHP code is enclosed within tags and can be embedded anywhere in an HTML document. A typical structure of a PHP script includes the following:
 

The PHP code between the tags is executed on the server, generating dynamic content that can be seamlessly integrated with HTML. 

PHP provides several control structures, including: 

a) Conditional statements (if, else if, else): These structures enable you to execute specific code blocks based on certain conditions. 

b) Loops (for, while, do-while, foreach): Loops allow you to repeat a block of code multiple times. They are useful for iterating over arrays, performing calculations, or executing tasks until a certain condition is met. 

c) Switch statement: The switch statement performs different actions based on different conditions. It evaluates an expression and executes the corresponding code block based on the matched case. 

Conditional statements in PHP help make decisions based on different conditions. The most common conditional statements are: 

a) If statement: The if statement executes a code block if a specified condition is true. Here's an example:
 

b) Else if statement: The else if statement allows you to test multiple conditions. It is used after the initial if statement and before the final else statement. Here's an example:
 

c) Ternary operator: The ternary operator provides a shorthand way to write simple if-else statements. It has the following syntax:
 

This will output "Adult" if the condition is true and "Minor" if it is false. 

Loops allow you to execute a block of code repeatedly. PHP provides the following loop types: 

a)for loop: The for loop executes a block of code a specific number of times. It consists of an initialisation, condition, and increment/decrement statement. Here's an example:
 

b) while loop: The while loop executes a code block as long as a specified condition is true. Here's an example:
 

c) Do-while loop: The do-while loop is similar to the while loop, but it guarantees that the block of code executes at least once, even if the condition is initially false. Here's an example:
 

d) forearch loop: The foreach loop iterates over arrays or other iterable objects. It automatically assigns the value of each array element to a variable. Here's an example:
 

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You can exit the loop prematurely with the help of the break statement. When encountered, it immediately terminates the loop and resumes execution at the next statement following the loop. It is often used with conditional statements to stop the loop based on a certain condition. 

The continue statement is used to skip the rest of the current iteration within a loop and move to the next iteration. It allows you to bypass certain code blocks and jump to the next iteration without executing the remaining statements. Both break and continue statements are typically used within loops to control the flow of execution based on specific conditions. 

In PHP, errors and exceptions can be handled using error-handling mechanisms. The error_reporting directive in PHP configuration settings controls the level of error reporting. You can set it to display, hide, or log errors to a file. Additionally, PHP provides the following error-handling functions and constructs: 

a) trigger_error(): Generates a user-defined error message. 

b) set_error_handler(): Sets a user-defined function as the error handler. 

c) try-catch block: Used for exception handling, where you can catch and handle specific exceptions thrown within a try block. 

d) throw statement: Throws a user-defined exception to be caught and handled by a catch block. 

Functions in PHP are blocks of reusable code that perform specific tasks. They help in organising code, promoting code reusability, and improving maintainability. Functions accept input parameters, perform operations on them, and can optionally return a value. They allow you to encapsulate and execute functionality whenever needed, reducing code duplication and enhancing modularity. Functions in PHP make code more readable, manageable, and efficient. 

Developers create user-defined functions to perform specific tasks as per their requirements. These functions are defined using the function keyword followed by a name, parameters (optional), and a block of code. User-defined functions provide flexibility and allow developers to extend PHP's functionality.  

Built-in functions, also known as native functions, are pre-defined functions provided by PHP itself. They are already available for use without requiring any additional code. PHP offers a vast collection of built-in functions for various purposes, such as string manipulation, array manipulation, mathematical operations, file handling, database interactions, and more. Examples of built-in functions include strlen(), array_push(), and file_get_contents(). 

To define a function in PHP, you use the function keyword followed by the function name, parentheses for parameters (if any), and a block of code enclosed in curly braces. Here's an example:
 

Function parameters in PHP allow you to pass values to functions for processing. Parameters are specified within the parentheses during function definition and are placeholders for values that will be used within the function's code. You can define multiple parameters separated by commas. For example: 

Return values in PHP enable functions to return a result after performing specific operations. The return statement specifies the value to be returned. Once a function encounters a return statement, it exits immediately, and the value is passed back to the calling code. For example:
 

Return values allow you to utilise the computed results for further processing or store them in variables. 

Recursion is a programming technique where a function calls itself during its execution. It involves solving a problem by breaking it into smaller, similar subproblems. Recursive functions have two essential components: a base case and a recursive case.  

The base case specifies the condition under which the function terminates, preventing infinite recursion. The recursive case defines the operations to be performed and calls the function itself with modified input parameters. 

Here's an example of a recursive function to calculate the factorial of a number: 

In this example, the factorial() function calls itself with a decremented value of $n until it reaches the base case ($n <= 1), where the recursion stops. The function then returns the product of $n and the factorial of $n-1. Eventually, the results are calculated and returned back up the recursive chain. 

By default, PHP passes function arguments by value, meaning the function receives a copy of the variable's value. However, you can pass arguments by reference to allow the function to modify the original variable directly. 

To pass an argument by reference, you prepend an ampersand (&) to the parameter declaration in both the function definition and function call. Here's an example:
 

In this example, the $num parameter is passed by reference using &$num. When the function modifies $num, it directly affects the original variable $value outside the function. Passing arguments by reference can be useful when you want to modify variables within a function and reflect those changes in the calling code. However, it should be used judiciously to avoid unexpected side effects and maintain code clarity.