What is SOLID in PHP?
Why Interviewers Ask This
This is a differentiating question used for senior and lead roles. Interviewers want to see if you can explain not just what happens, but why — and what the trade-offs are in different approaches.
Answer
SOLID is a set of five OOP design principles. S — Single Responsibility: a class should have one reason to change (one job). O — Open/Closed: open for extension, closed for modification — use interfaces and abstractions to add behavior without editing existing code. L — Liskov Substitution: subclasses must be substitutable for their parent classes without changing the correctness of the program — honor the parent's contract. I — Interface Segregation: prefer small, specific interfaces over large, general ones — clients should not be forced to implement methods they do not use. D — Dependency Inversion: high-level modules should not depend on low-level modules — both should depend on abstractions (interfaces). Following SOLID produces loosely coupled, easily testable, and maintainable PHP code.
Common Mistake
Rushing to answer is a common mistake. Take two seconds to structure your response: definition → example → trade-off. This structure makes complex PHP answers easy to follow.