A useful PHP interview answer explains behavior and tradeoffs, not only a definition. You may be asked why zero fails a loose check, how prepared statements work, or how to count repeated values without losing the original order. This reference combines those reasoning questions with practical exercises and a worked frequency counter. It covers language basics, object design, request handling, database correctness, and testing. Use it after the main course so you can connect each answer to working code. Runtime-sensitive features should be described with their version requirements rather than claiming every PHP installation supports the same syntax.
Explain types and comparisons with examples
A common question is the difference between == and ===. Loose comparison performs conversion rules, while strict comparison checks type and value. Explain why a filter result equal to integer zero must not be confused with false. Be ready to discuss missing array keys, null, isset(), and array_key_exists(): isset() returns false for a null value, while array_key_exists() can establish that the key exists. Show a concrete input rather than reciting 'strict is better' without context. Type declarations help express contracts, but coercion and strict_types behavior need careful explanation. Mention the caller-file rules and supported types when relevant. A strong answer states the intended data model and which comparison supports it.
Reason about arrays and algorithm cost
PHP arrays are ordered maps, which supports indexed-looking and associative usage. Know when keys are preserved, reindexed, or overwritten by array functions. A frequency counter can use values as keys, but PHP's key conversion rules can merge distinct representations unexpectedly. If the task requires arbitrary strings with exact identity, define a key encoding or another representation. Explain time and memory cost: one pass through n items is generally O(n) work with storage proportional to the distinct values, while nested scans can become O(n²). State input constraints, including maximum length and allowed types. Interview code should handle empty input and duplicates deliberately rather than only the sample list.
Discuss object design through responsibilities
Explain inheritance, interfaces, abstract classes, and traits by the responsibilities they support. An interface expresses a contract; inheritance shares an is-a relationship and implementation; a trait reuses methods with its own conflict rules. Composition often makes replaceable dependencies clearer than a deep inheritance tree. Discuss visibility, constructor invariants, and immutable data under the supported PHP version. Readonly properties and enums have version requirements and nuanced behavior, so state those instead of generalizing. A magic method can support a specific design but also hide errors and make static analysis harder. A good answer explains why the chosen mechanism helps this codebase and what cost it introduces.
Connect request handling to security boundaries
Describe the journey from a form or JSON body to validated data, an authorized operation, and a safe response. A field can be an array when you expect text, so input shape comes before string validation. Prepared statements protect SQL value boundaries, while HTML escaping protects output; neither establishes ownership. Session-authenticated changes need CSRF controls, and password storage uses password_hash() rather than a homegrown scheme. Uploads need bounded formats and private non-executable storage. If asked to make an AJAX endpoint, explain that JavaScript runs in the browser and PHP on the server, and fetch requires HTTP status checks. These answers show practical understanding rather than a list of attack names.
Explain transactions and concurrency
A transaction groups supported database changes, but isolation and query design determine concurrent behavior. Explain the stock race where two sessions both read one available item. An atomic conditional UPDATE can enforce stock availability better than an unchecked read-then-write. Keep parent and child inserts on one connection, retrieve the generated key there, and publish success after commit. MySQL DDL can commit implicitly, and generated IDs can have gaps after rollback. Retrying requires classification and safe duplicate handling. External email or payment is not automatically rolled back with the database. If the question is broader than your experience, state the boundary and outline what documentation or test you would consult.
Work through coding tasks clearly
Before writing code, clarify accepted inputs and outputs, constraints, and edge cases. Walk through one small example, then implement the simplest design that satisfies the rule. Name variables according to their role and separate pure calculation from external effects. Run or reason through empty input, a boundary, duplicates, and invalid input. State complexity and meaningful limitations without pretending the code solves a different larger problem. Tests should assert requirements independently rather than duplicate implementation formulas. For a production-style task, explain logging and error translation at the boundary, not in every helper. During review, fix a discovered bug openly and describe the corrected behavior. The ability to reason and communicate is part of the exercise.
Practice task and worked solution
Write a function that groups orders by customer and totals quantities, preserving the first-seen customer order. Define whether customer IDs are strings or integers and reject mixed invalid records rather than relying on accidental casts. The solution makes one pass, validates nonnegative integer quantities, and returns a documented result shape. Test empty input, repeated customers, zero quantity, and a malformed row. Explain time and memory cost. Then answer a design follow-up: if those orders came from a database, when could GROUP BY move aggregation into SQL, and what authorization filter must remain? Do not present an in-memory exercise as a replacement for all database reporting.
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Review functions, advanced OOP, and security.
The PHP language reference is the primary reference for types, comparisons, and object behavior.
PHP Interview Questions and Coding Exercises Examples
Each example shows the code, the output it produces and what each part does.
Count exact string values in one pass
This CLI exercise prefixes keys to avoid numeric-string key conversion. It accepts strings only and returns counts in first-seen order.
The two numeric-looking strings remain distinct. The function uses one pass with storage proportional to distinct strings, subject to ordinary hash-map behavior. An empty input returns an empty array.
<?php
function frequency(array $values): array {
$counts = [];
foreach ($values as $value) {
if (!is_string($value)) throw new InvalidArgumentException('Strings required');
$key = 's:' . $value;
if (!isset($counts[$key])) $counts[$key] = ['value' => $value, 'count' => 0];
$counts[$key]['count']++;
}
return array_values($counts);
}
foreach (frequency(['01', '1', '01', 'pen']) as $entry) {
echo $entry['value'] . ': ' . $entry['count'] . PHP_EOL;
}01: 2
1: 1
pen: 1PHP Interview Questions and Coding Exercises FAQ
Should I memorize every function?
No. Understand core behaviour, such as how arrays, strings, comparisons and errors work, and know how to consult the manual for exact argument orders. Interviewers value clear reasoning and safe habits over remembering whether the needle or the haystack comes first.
Is strict comparison always the whole answer?
Not on its own. Saying "use ===" is a good start, but explain the data contract and why it fits: what types the values should have, where they came from, and what should happen with unexpected input such as a numeric string from a form.
Should interview code perform real mail or writes?
No. Use controlled examples unless the task explicitly requires integration: a fake mailer, an in-memory array or a temporary SQLite database shows the design without side effects, and makes your solution easy to run and test.
What if a feature depends on a PHP version?
State the minimum and verify the exact behaviour. For example, say "str_contains needs PHP 8.0" or "enums need 8.1", and mention what you would use on older versions. Interviewers appreciate candidates who know where version boundaries matter.
PHP Interview Questions and Coding Exercises Exercises
Answer each one and check it instantly. If you get stuck, open the hint before the answer.
A task requires exact string identity for 01 and 1. What should the design preserve?
PHP Interview Questions and Coding Exercises Quiz
2 questions to check what you learned.
What distinguishes array_key_exists from isset for a null value?
- array_key_exists can report the present key while isset returns false
- Both always delete the key
- isset turns null into zero
- Neither accepts arrays
What should a coding solution explain besides sample output?
- Only the number of comments
- Input contract, edge cases, and complexity
- A promise of universal production readiness
- Unrelated framework slogans