Php Interview Questions & Answers for 2026

== is a loose comparison that performs type coercion before comparing — 0 == "foo" is true, "1" == 1 is true, null == false is true. === is a strict comparison that checks both value and type — 0 === "foo" is false. Always use === in application code to avoid unexpected bugs. The same applies to != vs !==. PHP 8 changed some loose comparison behaviours (0 == "foo" is now false) which broke code relying on old behaviour. In security-sensitive contexts like password comparison always use hash_equals() or password_verify().

Traits are a mechanism for code reuse in single-inheritance languages like PHP. A trait is like a partial class that can be mixed into any class with use TraitName. They can contain methods and properties but cannot be instantiated on their own. Use traits when multiple unrelated classes share behaviour — for example a Timestampable trait that adds created_at/updated_at logic, or a HasSlug trait. Avoid traits for behaviour that is tightly coupled to the class hierarchy — that is better served by inheritance or composition. Traits also support abstract methods, forcing implementing classes to define them.

PHP sessions store user data on the server and associate it with the user via a session ID in a cookie (PHPSESSID). session_start() reads or creates the session. Data stored in $_SESSION persists between requests. Security concerns: session fixation (attacker sets session ID before login — fix by calling session_regenerate_id(true) after login), session hijacking (steal the cookie — fix with HTTPS and httponly/secure cookie flags), and session expiry (set gc_maxlifetime and implement application-level timeout). Store minimal data in sessions and never store passwords.

PDO (PHP Data Objects) is a database abstraction layer that supports multiple databases (MySQL, PostgreSQL, SQLite) through a single API. It uses prepared statements natively which separate SQL structure from data, preventing SQL injection. Unlike mysqli which is MySQL-specific, PDO allows you to switch databases with minimal code change. Prepared statements also improve performance for repeated queries through query plan caching. Always use PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION to convert errors into exceptions rather than silent failures.

Closures are anonymous functions that can capture variables from the enclosing scope using the use keyword. Example: $multiplier = function($n) use ($factor) { return $n * $factor; }; Without use, closures cannot access outer scope variables (unlike JavaScript where closures automatically capture scope). Closures are instances of the Closure class and can be passed as callbacks, stored in variables, and used with array_map, array_filter, and usort. Arrow functions (fn($x) => $x * $factor) in PHP 7.4+ automatically capture outer scope variables.

Autoloading automatically includes class files when they are first used without requiring manual require statements. Composer generates an autoloader based on the autoload configuration in composer.json. PSR-4 maps namespaces to directory structures: a class App\Models\User maps to src/Models/User.php if configured as "App\\": "src/". Running composer dump-autoload regenerates the autoloader files. This enables the entire ecosystem of PHP packages to work without manual includes and is the foundation of modern PHP application structure.

include and require both insert and execute the specified file. require throws a fatal error if the file is missing; include only generates a warning and continues execution. include_once and require_once check if the file has already been included in the current request and skip it if so, preventing duplicate class or function definitions. In practice almost always use require_once for class and function files. Use include for template files where you can tolerate failure. Modern PHP with Composer autoloading makes manual includes largely unnecessary in application code.

Never concatenate user input directly into SQL strings. Always use PDO or mysqli prepared statements with parameterised queries: $stmt = $pdo->prepare("SELECT * FROM users WHERE email = ?"); $stmt->execute([$email]);. The database driver handles escaping, making injection impossible regardless of input content. Additional defences: validate and whitelist input types (cast to int for numeric IDs), use least-privilege database users, and never display raw database errors to users in production. PDO with prepared statements is the primary and sufficient defence.