Top PHP Interview Questions and Answers

PHP is a server-side scripting language most commonly used for web development to create dynamic and interactive web pages. It is executed on the server and is often used in web applications like content management systems (CMS) and e-commerce sites, and other custom web applications. Notable websites built with PHP include Facebook and WordPress.
It works well with HTML, which is why it's been a go-to choice for backend web development for years. We can also use it for creating login systems, handling forms, connecting to databases, and making REST APIs.

Example

PHP is open-source, hence free. It works on Windows, Linux, and macOS since it is cross-platform. It’s easy to learn PHP as it has a clean syntax. PHP runs well for websites as it is fast. It has good database support for MySQL, PostgreSQL, and more. Being a server-side scripting language, the code executes on the server instead of the browser. To manage user data across pages, PHP has a session handling and cookie handling feature.
 

== checks if values are equal, even if types are different.
=== checks both value and type.

For example,

echo is faster, can output multiple values.

Print always takes a single argument rather than multiple comma-separated arguments, as echo does. As print returns 1, it can be used in expressions.

Example:

An indexed array in which keys are numbers.
Associative array where keys are strings.
A multidimensional array in which arrays are inside arrays.

Example:

GET sends data in the URL (visible). Good for search or filters.
POST sends data in the request body (hidden). Safer for forms.

A session in PHP is a way to persist user data across multiple web pages. Sessions keep track of certain user activities (e.g., logged-in status, contents of a shopping cart, etc.) as a user browses a website. PHP sessions persist these user actions on the server side, which is more secure and provides consistent user interactions during a single visit.

Example:

A cookie stores small bits of data in the user's browser.

Example:

php.ini is the main configuration file for PHP. It controls file upload size, Error reporting, memory limits, and default timezone. Changes in this file affect how PHP behaves.

include() adds a file. If the file is missing, it shows a warning but continues running.
require() also adds a file, but if it's missing, it stops the script with a fatal error.
include_once() adds the file only once. If it's missing, it shows a warning but continues.
require_once() adds the file only once. If it's missing, the script stops.

Example:

Magic methods are special functions in PHP that start with double underscores. They run automatically during certain actions in a class.
__construct() runs when an object is created.
__call() runs when calling a method that doesn’t exist.
__toString() runs when you try to print the object as a string.

Example:

PHP connects to MySQL using extensions like mysqli or PDO. You can send queries to insert, update, read, or delete data.

Example:

header() is used in PHP to send raw HTTP headers to the browser before any output. It can redirect pages, set content types, or control caching.

Example:

To keep user data across pages, you start a session with session_start(). To end it, use session_destroy().

Example:

Traits are a mechanism for code reuse in PHP that allows you to share methods across multiple classes, without inheritance. Traits help eliminate code duplication because they allow different classes to utilize the same set of functions. Traits are useful when multiple classes need the same functionality but don’t share the same parent class.

Example:

An abstract class can have both methods with and without a body. Interfaces only have method names but no body.
Use an abstract class when classes share common code.
Use an Interface when different classes must follow a contract.

Output
Eating
Bark
Phone On
 

ini_set() lets you change PHP settings at runtime.

Example:

PEAR (PHP Extension and Application Repository) is a collection of reusable PHP code libraries. It helps developers avoid writing common features from scratch.

isset() checks if a variable is set and not null.
empty() checks if a variable has a false-like value (like 0, "", or null).

Use the php command followed by the file name.

Example:

PDO (PHP Data Objects) is a built-in PHP extension that lets you connect to different databases using the same code. It helps write cleaner, safer, and more flexible database queries.

You’d use PDO to prevent SQL injection and support multiple databases like MySQL, SQLite, and PostgreSQL with the same codebase.

Composer is a tool for managing PHP project dependencies. If your project needs a library, Composer installs it and keeps it updated.
composer require monolog/monolog
This command adds Monolog (a logging tool) to your project, saving time and keeping code organized.
 

PHP dependency injection is a design pattern that allows you to provide the required objects (dependencies) to a class from the outside rather than creating them internally. Code that is dependent on other classes is more flexible, testable, and maintainable than code that tracks all of the dependencies itself. Effectively, dependency injection is accomplished through constructor injection, method injection, and dependency injection containers for PHP.

Anonymous functions are functions without a name. Closures are anonymous functions that can access variables outside their scope. They're useful in callbacks and array functions.

Access modifiers control where class properties and methods can be accessed. They help with security and code structure.
public: accessible everywhere
protected: accessible in the class and subclasses
private: only inside the same class

An autoloader loads class files automatically when you use them, so you don’t need to write require for every file. This keeps your code clean and avoids manual includes.

PSR (PHP Standard Recommendation) is a set of coding guidelines. It helps PHP developers write code that’s consistent and compatible with others.
For example, PSR-4 defines how to autoload classes from file paths, and PSR-12 covers coding style.
 

PHP handles memory using a built-in garbage collector. It frees memory by removing variables and objects no longer in use. This is helpful in long scripts or loops.
You can use unset() to manually clear variables:
 

SQL Injection: Use prepared statements with PDO
XSS: Escape output using htmlspecialchars()
CSRF: Use CSRF tokens in forms
File uploads: Validate file types and sizes

Always sanitize user input and validate server-side.
 

Namespaces prevent name conflicts by grouping related classes. They help organize large projects and avoid class name clashes.