44 PHP interview questions with answers

All PHP interview questions

• What is PHP and what does it stand for?

  PHP is a recursive acronym for PHP: Hypertext Preprocessor, meaning the first P stands for PHP itself. The name originally stood for Personal Home Page in the first versions of the 1990s, and was later changed to this recursive form. PHP is a server-side language…

• What are PHP superglobals? Give examples.

  Superglobals are pre-defined variables available everywhere in a script (including inside functions and methods) without declaring global. PHP populates them automatically on every request. The most important ones: - $GET: query string parameters from the URL (?page=2). - $POST: form data sent in the request…

• What's the difference between $_GET and $_POST?

  Both collect form data but differ in where the data lives and how much you can send. - $GET reads from the URL query string, so the data is visible (http://example.com?name=John). URL length is limited by the browser and the web server (usually a few…

• What are PHP data types and what are the main ones?

  PHP has 8 core data types in 3 groups: 4 scalar types (hold a single value), 2 compound types (hold many values) and 2 special types. - Scalar: string, int, float, bool. - Compound: array (a list or key-value pairs), object (an instance of a…

• What is the difference between == and === operators?

  Both are comparison operators but with different strictness. == checks if values are equal but allows type conversion. For example, "5" == 5 returns true because PHP converts the string to a number. The === (strict equal) checks both value AND type, so "5" ===…

• How do you work with arrays in PHP? What are the main types?

  Under the hood PHP has only one array type: an ordered map (key → value pairs that keep insertion order). The "three array types" people mention are just three ways of using it: - Indexed array: integer keys auto-assigned from 0, accessed by position: $fruits[0].…

• What are common string functions in PHP?

  Commonly used string functions: - strlen(): string length (in bytes; for UTF-8 text with accents use mbstrlen()). - strpos(): position of the first occurrence of a substring, false if not found. - strreplace(): replace text. - substr(): extract a portion of a string. - strtolower(),…

• What is the difference between include and require?

  Both pull another PHP file into the running script. They differ in what happens when the file is not found. - include: raises a warning (EWARNING), returns false, and the script keeps running. - require: also raises the warning first, then stops the script with…

• How do you handle form data in PHP?

  Access form data through superglobals: for GET use $GET['fieldname'], for POST use $POST['fieldname']. The rule is validate the raw input when it comes in, escape it when you output it: - Check it exists: $POST['email'] ?? '' (or isset()) avoids a warning when the field…

• What are PHP error types and how are they handled?

  PHP has two error mechanisms: traditional errors (Notice, Warning, Fatal error) that are only printed or logged, and exceptions since PHP 7 that can be caught with try/catch. Traditional errors are split by severity: - Notice / Warning: the script keeps running. Examples: reading an…

• How do you define and use constants in PHP?

  Define constants with define("CONSTANTNAME", value) or const CONSTANTNAME = value;. Constants have no dollar sign, cannot be changed after definition, and are global without needing the global keyword. Since PHP 8.0, constants are always case-sensitive (the $caseinsensitive parameter of define() was removed). Use constants for…

• What are classes and objects in PHP? What's the difference?

  A class is a blueprint: it defines which properties and methods an object will have. An object is a concrete instance created from that class with new. One class can create many objects. The two new calls create two independent objects. Each object has its…

• What is inheritance in PHP?

  Inheritance lets a child class receive properties and methods from a parent class, promoting code reuse. Use class Admin extends User to inherit. The child can access the parent's public and protected members (not private), override parent methods, add new functionality, and call the parent's…

• Does PHP support multiple inheritance? How do you achieve similar functionality?

  PHP does not support multiple inheritance: a class can extends only one parent. To get a similar effect, PHP splits the job across two tools: interfaces for contracts (which methods a class must have) and traits for code reuse. Employee implements two interfaces at once,…

• What are interfaces and when should you use them?

  An interface is a contract: a list of methods (name, parameters, return type) that every class implementing it must have. An interface contains no implementation code. If InMemoryUserRepository misses a method of the interface, PHP raises a fatal error as soon as the class is…

• What are Traits in PHP and how do they solve problems?

  Traits let you reuse methods in multiple unrelated classes without inheritance. PHP allows only one parent class, so when User and Product both need logging but share no parent, a trait is where that shared code goes. Think of traits as "horizontal" reuse versus inheritance's…

• What are magic methods in PHP? Give examples.

  Magic methods are special methods (names starting with ) that PHP calls automatically in response to certain actions. Key ones: - construct(): runs when an object is created. - destruct(): runs when the object is destroyed. - get(): reading a property that does not exist…

• What is the difference between static and instance methods?

  An instance method belongs to an object, can use that object's own data through $this, and is called with $object-method(). A static method belongs to the class itself, has no $this, and is called with ClassName::method(). getName() needs to know "which user", so it must…

• What are access modifiers (public, private, protected)?

  Access modifiers control visibility of properties and methods: public accessible everywhere (inside/outside class and subclasses), private accessible only inside that class (not subclasses), protected accessible inside class and subclasses but not outside. Example: private $password prevents direct access like $user-password = "hacked", forcing use of…

• What is PDO and how does it prevent SQL injection?

• How do PHP sessions work and what's the difference from cookies?

• What are namespaces and why should you use them?

• What is autoloading and how do you implement it?

• What are generators in PHP and when do you use them?

• What is the difference between unset() and unlink()?

• How do you prevent Cross-Site Scripting (XSS) attacks in PHP?

• How do you handle exceptions in PHP?

• What is type hinting and why is it important?

• What are abstract classes and when do you use them?

• What are Fibers in PHP 8.1 and how are they different from threads?

• What are the constraints of readonly classes introduced in PHP 8.2?

  A readonly class (PHP 8.2) adds readonly to every declared property, whether public, protected or private, and forbids dynamic properties. Each property can be assigned only once, from inside the class; any later reassignment throws an Error: Constraints: - Every property must be typed (no…

• What are Enums in PHP 8.1 and when should you use them instead of constants?

  An enum (PHP 8.1+) is a type with a fixed set of values. Each value is a single object of the enum, so comparison with === and match both work. Two kinds of enum: - Pure enum: only case names; no -value, no from()/tryFrom(), and…

• What are Value Objects in PHP and when should you use them instead of primitives?

• Why store passwords with `password_hash()` / `password_verify()` instead of `md5`/`sha1`?

  md5/sha1 are fast, unsalted hashes → trivial to brute-force on a GPU and to reverse via rainbow tables. passwordhash() uses a deliberately slow algorithm (bcrypt by default, or Argon2) with a random salt embedded in the output string, so you never need a separate salt…

• What is a nullable type (`?type`) in PHP?

  ?T declares a value that is either type T or null. It works for parameters, return types, and properties (since 7.4). It is an explicit contract: ?int accepts int or null, but not a string. - Since PHP 8, ?T is equivalent to the union…

• What are streams and stream wrappers in PHP?

• What is OPcache and how does it optimize PHP performance?

• What principles make up SOLID (applied in PHP)?

  Five OOP design principles distilled by Robert C. Martin: - S — Single Responsibility: a class should have one reason to change. Keep email-sending logic out of the User class. - O — Open/Closed: open for extension, closed for modification. Add a new PayPalGateway class…

• What is Composer and how does Laravel use it?

  Composer is PHP’s per-project dependency manager (the role npm plays for Node). It resolves the dependency tree and generates an autoloader. - composer.json — declares packages and semver version constraints (^8.0, ~2.3). - composer.lock — pins the exact installed versions for reproducible builds. Commit it;…

• How does PHP 8's `match` differ from `switch`?

  Four differences worth naming: 1. Strict comparison. match uses ===, switch uses ==. This is the follow-up interviewers push on. 2. It is an expression. match returns a value, so you can assign or return it directly. switch is a statement. 3. No fall-through. No…

• What problems do PHP 8 named arguments and constructor property promotion solve?

  Constructor property promotion declares and assigns properties directly on constructor parameters, removing the private X $x; ... $this-x = $x; boilerplate. Named arguments pass parameters by name instead of by position: Benefits: calls with many boolean/null parameters become readable, and you can skip optional parameters…

• What are PHP 8 attributes? How do they differ from docblock annotations, and how do frameworks use them?

• Why is the Singleton pattern less valuable in PHP (shared-nothing, re-initialized on every request) than in Java (long-running JVM with shared memory)?

  In traditional PHP (PHP-FPM, modphp), each request starts with empty memory and every object is destroyed when it ends — so a Singleton is only "single" within one request. It cannot keep a connection, cache or state for the next request, which is exactly why…

• How do empty() and isset() differ in PHP?

  isset($x) returns true when the variable exists and is not null. empty($x) returns true when the variable does not exist or its value is falsy — equivalent to !isset($x) $x == false. Both are language constructs and raise no warning for undefined variables. Value isset()…