PHP Developer OOPS Concepts Interview Guide

Choose composition when the relationship is about using a capability rather than being substitutable for the base type. Delegate behind a small interface and keep invariants local. Inheritance can be appropriate, but test that derived behavior honors the expectations of callers using the base abstraction.

Check your understanding: Replace the implementation behind an interface and run the same contract tests.

Common trap: Using inheritance only to avoid copying a few lines.

Concept reference: Dev.java: language, collections and JVM learning paths

Not necessarily. Encapsulation protects meaningful invariants and exposes valid operations, rather than merely making fields private. Model state transitions so callers cannot construct invalid combinations, and keep representation details from leaking through mutable references.

Check your understanding: Attempt an invalid transition through the public API.

Common trap: Equating private fields with a complete domain model.

Concept reference: Dev.java: language, collections and JVM learning paths

Look for implementers that must provide meaningless operations or callers that depend on methods they never use. Split responsibilities around real usage while avoiding one interface per line of code. Test consumers against alternate implementations to expose hidden assumptions.

Check your understanding: Implement a second legitimate provider without no-op methods.

Common trap: Designing an interface from one implementation’s internals.

Concept reference: Dev.java: language, collections and JVM learning paths

Binding values keeps them separate from SQL structure and avoids relying on hand-built quoting rules. Use the database driver’s supported parameter API, validate input constraints and allowlist dynamic identifiers separately. A prepared value placeholder cannot safely stand for an arbitrary table name or SQL clause.

Check your understanding: Test quoted input and an unsupported dynamic sort column.

Common trap: Using parameter binding for values but concatenating untrusted identifiers.

Concept reference: PHP: preventing SQL injection

Prevent the injection first, then reduce the application account’s database permissions to necessary operations. Avoid exposing database error details and separate operational credentials from the web request path. Layered controls reduce impact but do not make unsafe query construction acceptable.

Check your understanding: Verify the application account cannot perform an unnecessary administrative operation.

Common trap: Relying on restricted permissions instead of fixing unsafe SQL.

Concept reference: PHP: preventing SQL injection

Return a consistent documented error contract and a status appropriate to the failure class. Do not report success with an embedded failure flag that callers may miss. Keep internal stack traces and credentials out of responses, and define retry behavior separately from user-correctable validation errors.

Check your understanding: Test invalid input, forbidden access and a failed downstream connection.

Common trap: One generic successful response for every outcome.

Concept reference: IETF RFC 9110: HTTP semantics

First, identify technical constraints and define measurable service objectives. Next, follow a request across validation, business logic, storage and asynchronous work. Contrast architectural trade-offs across reuse, clarity, extensibility and accidental complexity, explicitly mitigate the risk of a large payload blocks the critical interaction, and confirm system stability using a network waterfall, interaction timing and a constrained-device test.

Common trap: Reaching for a specific library or framework before defining constraints, failure envelopes, and automated verification criteria.

Prioritise the failure mode exhibiting the highest user blast radius and lowest observability. Formulate an explicit containment boundary, implement idempotent retries with jitter, and establish an automated rollback threshold. Verify resilience through an accessibility audit, keyboard trace and corrected acceptance test.

Common trap: Relying on passive monitoring dashboards without defining explicit error-budget alerts, rollback triggers, and verified recovery procedures.

Structure the response using Context-Decision-Tradeoff-Result: articulate the business and technical constraints, compare viable alternatives, explain the implementation (follow a request across validation, business logic, storage and asynchronous work), and document the accepted trade-off. Provide concrete proof: a focused unit test, dependency boundary and refactoring comparison.

Common trap: Speaking only in high-level abstractions or team accomplishments without detailing your direct implementation decisions, trade-offs, and measured results.

Formulate a falsifiable hypothesis from observable telemetry before altering configurations. Then correlate the request trace with transaction state, queue ownership and dependency budgets. Isolate the defect to the smallest reproducible boundary, validate root cause with evidence, and confirm full resolution using a compatibility test matrix and a staged rollout metric.

Common trap: Applying speculative fixes or restarting services blindly without establishing an observable signal connected to a falsifiable hypothesis.

Produce an idempotency test, failure trace and repeatable recovery runbook. Document baseline assumptions, technical mechanism (follow a request across validation, business logic, storage and asynchronous work), rejected alternatives, bounded failure envelopes, and deterministic pass criteria. Supply reproducible verification via lease-expiry tests, idempotency records and a restart recovery drill.

Common trap: Presenting architecture diagrams or slides lacking automated unit/integration tests, observable metrics, or automated rollback configurations.