PHP Interview Questions

PHP originally stood for Personal Home Page, but it's now a recursive acronym for PHP: Hypertext Preprocessor, reflecting its evolution from a simple scripting tool into a full server-side language.

PHP variables start with a dollar sign followed by the variable name, and they don't require explicit type declaration since PHP is a loosely typed language. For example, $name = "John"; creates a string variable.

PHP supports scalar types like integer, float, string, and boolean, compound types like array and object, and special types like null and resource.

Both output text to the screen, but echo can take multiple comma-separated arguments and has no return value, while print takes a single argument and always returns 1, which lets it be used inside an expression.

== checks for value equality after type coercion, so "5" == 5 is true, while === checks both value and type, so "5" === 5 is false since one is a string and the other an integer.

An associative array uses named keys instead of sequential numeric indexes, letting you access values by a meaningful key, like $person['name'] instead of $person[0].

Both insert the content of another PHP file, but require throws a fatal error and stops script execution if the file isn't found, while include only throws a warning and lets the script continue running.

A superglobal is a built-in variable that's always accessible in every scope throughout a script, without needing to be passed around. Examples include $_GET, $_POST, $_SESSION, and $_SERVER.

isset() checks whether a variable is set and is not null, commonly used to safely check for the presence of form data or array keys before using them, avoiding undefined variable notices.

isset() returns true if a variable exists and isn't null, while empty() returns true if a variable doesn't exist, is null, or holds a falsy value like an empty string, zero, or false.

A function is a reusable block of code defined with the function keyword, optionally taking parameters and returning a value. For example, function add($a, $b) { return $a + $b; } defines a simple addition function.

Variable scope determines where a variable is accessible in the code. Variables declared inside a function are local to that function by default and aren't visible outside it unless explicitly passed or declared global.

The global keyword lets a function access a variable defined outside its local scope, in the global scope, by referencing that same variable name inside the function rather than creating a separate local copy.

A class is a blueprint defining properties and methods, while an object is an instance created from that class using the new keyword. For example, class Car {} defines the blueprint, and $myCar = new Car(); creates an object.

public members are accessible from anywhere, private members are accessible only within the defining class, and protected members are accessible within the class and any subclasses that extend it.

A constructor is a special method named __construct that runs automatically when an object is created, typically used to initialize the object's properties with starting values.

Inheritance lets a class, called a child or subclass, extend another class, called a parent, using the extends keyword, inheriting its properties and methods while being able to add or override its own.

An array is a compound data type holding multiple values, indexed or keyed, while a string is a scalar type holding a sequence of characters. PHP does provide functions to convert between the two, like implode and str_split.

$_SESSION stores user-specific data across multiple page requests, using a session identifier typically stored in a cookie, letting you persist information like a logged-in user's ID between page loads.

GET appends form data to the URL as query parameters, making it visible and limited in length, while POST sends data in the request body, better suited for larger or sensitive data like passwords.

foreach is the most common way to loop through an array in PHP, iterating over each key-value pair without needing to manually manage an index, for example foreach ($items as $key => $value) { }.

Type juggling is PHP's automatic conversion of a value from one data type to another based on context, such as converting a numeric string to an integer when used in arithmetic. It's a consequence of PHP being loosely typed.

The null coalescing operator, written as ??, returns its left operand if it exists and isn't null, otherwise it returns the right operand, commonly used as a shorthand for checking isset() with a fallback value.

An interface defines a contract of method signatures that any implementing class must provide, without including any implementation itself. A class uses the implements keyword to fulfill an interface's contract.

A function is a standalone block of reusable code, while a method is a function defined inside a class and associated with objects of that class, typically called on an object instance.

An abstract class cannot be instantiated directly and may contain abstract methods, meaning method signatures without an implementation, that any concrete subclass must implement.

array_map() applies a given callback function to every element of an array and returns a new array containing the results, useful for transforming data without writing an explicit loop.

array_filter() returns a subset of an array containing only elements that pass a given callback test, while array_map() transforms every element and returns an array of the same length with the results.

A namespace groups related classes, functions, and constants under a common prefix to avoid naming collisions, especially useful in larger applications or when combining code from multiple libraries.

Composer is PHP's dependency manager, letting developers declare the libraries a project needs in a composer.json file and automatically download and autoload them, rather than manually managing external code.

A try-catch block lets you handle exceptions gracefully, running the code inside the try block and catching any thrown exception in the catch block instead of letting the script crash with an unhandled error.

A static method belongs to the class itself rather than any specific object instance, called using the class name and the double colon operator, like ClassName::methodName(), without needing to create an object.

require_once checks whether the file has already been included before including it again, preventing duplicate declarations, while require will include the file every time it's called even if it was already included.

A constant is a name for a value that doesn't change during script execution, defined using the define() function or the const keyword, such as define('SITE_NAME', 'MyApp');.

json_encode() converts a PHP array or object into a JSON string, commonly used for API responses, while json_decode() parses a JSON string back into a PHP array or object.

I would validate both on the client side for immediate feedback and on the server side as the authoritative check, since client-side validation can be bypassed. I'd use PHP's filter_var() functions for common patterns like email validation and sanitize input before using it.

I would enable error reporting to see exactly which line and variable triggered the notice, then trace back to confirm the variable is being set in every code path that reaches that line, often adding an isset() check or a default value if the variable is conditionally set.

I would regenerate the session ID after login to prevent session fixation attacks, set the session cookie to HttpOnly and Secure flags, and set a reasonable session timeout rather than letting sessions persist indefinitely.

I would use SQL's LIMIT and OFFSET clauses to fetch only the records needed for the current page, along with a separate count query to calculate total pages, rather than pulling the entire dataset into PHP and slicing it in memory.

I would validate the file type and size on the server side, beyond just checking the file extension, also verifying the actual MIME type, and store uploaded files outside the publicly accessible web root or rename them to prevent path traversal or execution of malicious scripts.

I would look for an N+1 query pattern, where a query runs once per loop iteration instead of fetching all needed data in a single batched query, and rewrite it to use a join or an IN clause instead.

I would disable displaying errors directly to users in production, since that can leak sensitive information, and instead log errors to a file or monitoring service using error_log() or a logging library, so issues can be investigated without exposing internals to visitors.

I would set the appropriate Content-Type header to application/json, parse the incoming request method and body, validate the input, and return a properly structured JSON response along with an appropriate HTTP status code for success or failure.

I would use password_hash() with the default bcrypt or argon2 algorithm to hash passwords before storing them, and password_verify() to check a submitted password against the stored hash, never storing plain text or using a fast, reversible hash like MD5.

I would look for opportunities to use early returns to reduce nesting, or extract complex conditions into well-named boolean functions, since deeply nested conditionals are hard to read and prone to logic errors during future changes.

I would escape any user-supplied data before rendering it in HTML using htmlspecialchars(), so that characters like angle brackets can't be interpreted as HTML or script tags by the browser.

I would evaluate whether an in-memory cache like Redis or Memcached fits the use case, storing computed results with an appropriate expiration time, and falling back to recomputing only when the cache misses.

I would read and process the file line by line using fgetcsv() in a loop rather than loading the entire file into an array at once, which keeps memory usage constant regardless of file size.

I would track request counts per client, keyed by IP address or API key, in a fast store like Redis with a sliding or fixed time window, rejecting requests that exceed the configured threshold with a 429 status code.

I would store all timestamps in UTC in the database and convert to the user's local timezone only at display time, using DateTime and DateTimeZone objects, rather than storing local times that become ambiguous across regions.

I would use dependency injection to pass the database connection into the class rather than hardcoding it, then use a mocking framework like PHPUnit's mock objects to substitute a fake connection during tests, isolating the class's logic from the actual database.

I would migrate the code to use PDO or mysqli, since the old mysql_* extension was removed entirely in PHP 7, mapping each function call to its equivalent while also taking the opportunity to switch to prepared statements if the old code was vulnerable to SQL injection.

I would use database-level locking, such as a transaction with a row lock or a unique constraint, rather than trying to handle concurrency purely in PHP application code, since PHP's typical request lifecycle doesn't share state between concurrent requests by default.

I would review the changelog for breaking changes, like removed functions or stricter type checking, run the existing test suite against the new version, and use a tool that scans for deprecated function usage before switching the production environment over.

I would move data access and business rules into dedicated classes or services, keeping the templates or view files focused purely on rendering output, which makes the logic testable independently of how it's displayed.

I would apply context-specific escaping, using htmlspecialchars() for HTML output, prepared statement parameter binding for database queries, and urlencode() for URL components, since a single generic sanitization approach doesn't protect against every type of injection.

I would check where memory usage grows unexpectedly, often from accumulating data in an array across many loop iterations, and consider unsetting variables no longer needed or processing data in smaller batches instead of loading everything at once.

I would use environment variables loaded through something like a .env file and a library that reads them, keeping sensitive credentials out of version control, and having each environment's config values injected at deploy time rather than hardcoded in the codebase.

I would check for resource exhaustion first, like hitting the PHP-FPM worker pool limit or database connection pool limit under concurrent load, since those issues often only surface under real traffic patterns that a local single-request test doesn't replicate.

I would weigh the bug-prevention benefit of declare(strict_types=1) against the migration effort, since strict typing can surface previously silent type coercion bugs, but flipping it on a large codebase all at once risks breaking code that relied on loose typing behavior. A gradual, file-by-file rollout with test coverage tends to be safer.

I would use a queue system, like a message broker backing a job queue, so long-running tasks don't block the request-response cycle, and design jobs to be idempotent so a retry after a failure doesn't cause duplicate side effects.

I would tune the process manager mode and the number of child processes based on available memory and average request memory footprint, and monitor for a growing request queue, which signals the pool is undersized for current traffic.

I would enforce tenant isolation at the data access layer, beyond just application logic, so a bug in one code path can't accidentally leak data across tenants, and keep authorization checks centralized rather than scattered across individual endpoints.

An ORM speeds up development and reduces boilerplate for common CRUD operations, but it can generate inefficient queries for complex reporting needs, so I'd typically use the ORM for standard operations and drop to raw SQL or query builder methods for performance-critical or complex queries.

I would look for objects or closures holding references that prevent garbage collection, since long-running PHP processes, unlike typical short request cycles, can accumulate memory from patterns that would otherwise get cleaned up automatically between requests.

I would apply different cache expiration strategies based on data volatility, using short or event-based invalidation for frequently changing data and longer TTLs for stable reference data, rather than applying one blanket caching policy across all data types.

I would identify natural seams in the existing codebase, like modules with few dependencies on the rest of the system, and extract those first as a proof of concept, rather than attempting a full rewrite, since incremental extraction lets you validate the approach with lower risk.

I would look at whether classes require concrete implementations of unrelated modules just to be instantiated, which usually signals missing interface abstractions, and consider introducing dependency injection to decouple classes from concrete dependencies they don't actually need to know about.

I would use a profiling tool like Xdebug's profiler or a production-safe APM tool to identify which function calls or database queries consume the most time, rather than guessing based on code review alone, since actual bottlenecks are often not where you'd expect.

I would look at whether the codebase's structure is actively slowing down new feature delivery or causing repeated production incidents, since a rewrite is rarely justified purely on code aesthetics but often is when delivery velocity has measurably degraded due to accumulated debt.

I would frame the risk in terms of security exposure, since unsupported versions no longer receive security patches, and quantify the growing difficulty of hiring engineers experienced with increasingly outdated tooling, both of which resonate with leadership beyond a purely technical upgrade argument.

I would require checking a package's maintenance activity, security advisory history, and dependency footprint before approval, and maintain a lightweight internal registry of pre-approved packages so teams aren't re-evaluating the same common libraries independently.

I would prioritize documentation and pairing on the highest-risk, most business-critical parts of that codebase first, since concentrated knowledge risk compounds the longer it goes unaddressed and becomes far costlier to unwind after that person leaves than to address proactively.

I would establish a shared baseline, like requiring Composer lock files be committed and automated dependency vulnerability scanning in CI, while leaving room for each application to choose specific libraries suited to its own needs.

I would tier applications by exposure and business criticality, requiring the fastest patch turnaround for public-facing applications handling sensitive data, while allowing a slightly longer, still bounded window for internal, lower-risk tools.

I would weigh the hiring and cross-team mobility benefit of standardization against the disruption of migrating applications that are working well on a different framework, generally standardizing for new projects while not forcing a costly migration on stable existing ones.

I would assess each legacy application's business criticality and maintenance cost, sunsetting or consolidating low-value, high-maintenance-burden applications while continuing focused investment in ones still core to the business.

I would generally favor framework conventions since they lower onboarding cost and align with community documentation, deviating only where the application's specific scale or domain genuinely requires it, rather than customizing preemptively based on anticipated future needs that may never materialize.

I would quantify the current cost of manual regression testing and production incidents traceable to insufficient coverage, since that concrete cost, compared against the investment needed to build a solid test suite, usually makes the case clearer than an abstract quality argument.

I would define clear, measurable thresholds, like maximum response time at a given percentile, tied to automated monitoring and alerting, rather than leaving performance as an informal, subjective judgment left to individual teams to interpret differently.

I would look at whether deployment frequency and rollback speed meet the business's actual delivery needs, since a hosting model that made sense at a smaller scale can become a bottleneck as release cadence and traffic grow, warranting a reassessment rather than continuing on inertia.

I would require a deprecation period with clear migration guidance before removing old functionality, along with semantic versioning so consuming applications can control when they adopt breaking changes, rather than forcing every dependent application to update simultaneously.

I would start by identifying where inconsistent practices have caused real friction across teams, like incompatible coding standards slowing code review, then establish shared tooling and guidelines addressing those specific pain points rather than imposing a theoretical best-practices document from the top down.

I frame technical debt in terms of the specific feature delivery it's already slowing down, using a recent concrete example, since leadership responds better to a demonstrated velocity cost than an abstract code quality argument they can't easily evaluate.

I would move from informal tribal knowledge toward documented standards and automated enforcement through CI, since practices that worked when a handful of developers could just ask each other directly don't scale once no single person has visibility into every team's codebase.

I try to ground the disagreement in specific, concrete future requirements rather than abstract architectural preference, since experienced engineers often converge once they're evaluating the same specific scenario. If it doesn't resolve, I document the tradeoff and make the call rather than letting the disagreement stall delivery.

I would require design decisions for significant systems to be documented with their rationale, beyond just their implementation, and rotate ownership periodically so more engineers build deep familiarity rather than leaving critical systems understood by only one or two people.

I present the specific risk of shipping with reduced testing in terms of likely production impact, and let leadership make an informed tradeoff, rather than silently absorbing the risk or unilaterally blocking the deadline without giving them the full picture.

I would identify engineers who already show strong judgment about tradeoffs, beyond just technical depth, and deliberately involve them in cross-team architectural decisions well before a transition is needed, since that judgment is the hardest part of the role to build quickly under pressure.

I focus on making sure the underlying engineering principles, like separation of concerns and test coverage expectations, transfer even as specific tools change, since teams that treat new tooling as a reason to abandon established principles tend to relearn the same lessons the hard way.

I would present a clear timeline of what happened, the root cause, the immediate fix, and the longer-term prevention plan, rather than a vague reassurance that it's been handled, since executives need enough concrete detail to assess business exposure and communicate confidently to their own stakeholders.

I would look at whether review comments are catching genuine correctness or maintainability risks versus mostly debating style preferences that automated tooling could catch instead, since a review process dominated by stylistic nitpicking isn't earning its cost in developer time.

I would quantify the duplicated effort currently spent by individual teams solving the same infrastructure problems independently, since that redundant cost, made visible, usually makes a stronger case than an abstract efficiency argument about shared services.

I try to separate what's genuinely unique about their constraints from what's just unfamiliar or inconvenient, since resistance sometimes stems from habit rather than real incompatibility. Where the constraint is genuinely real, I'd rather document an explicit exception than force an ill-fitting standard.

I would push the organization to adopt modern PHP language features incrementally on new code while being deliberate about legacy code migration pace, since PHP's ecosystem has matured substantially, and organizations that keep writing code as if it were an older PHP version miss real maintainability gains available to them now.