A PHP developer with 3 years of experience should know more than just the PHP syntax. At this stage the developer would have worked a real application, dealt with databases, fixed bugs, worked with APIs, used a framework, and collaborated with other developers.
You might be asked to explain PHP concept and write a small code, troubleshoot problem or explain how to approach real-world issue. The article on what is expected from 3 years experience PHP developer explains basic requirement. Here we will discuss more technical questions.
1. Core PHP Interview Questions
A developer with three years of experience should be able to understand core concepts and explain basic differences.
Question 1: What is the difference between == and === in PHP?
This is a simple question, but it tests whether the candidate understands PHP’s type comparison behavior.
A good answer should explain that == performs a loose comparison and may convert types, while === checks both the value and the type.
For example:
5 == "5" // true
5 === "5" // false
Experienced developer should understand why strict comparison is often preferable when the type matters with real-world examples.
Question 2: What is the difference between include and require?
A candidate should understand what happens when PHP loads another file and what happens when that file cannot be loaded.
They should also know the difference between the _once versions and why repeatedly loading the same file can cause problems.
Question 3: What are namespaces in PHP and why are they useful?
Namespaces help prevent naming conflicts between classes, functions and constants.
Your answer should explain how namespaces are useful when working with large applications and integrating with third party packages. It helps avoid conflict with many classes that can potentially stop the execution of program due to simple name conflict.
Question 4: What are traits in PHP?
The candidate should explain what a trait is and why it is useful for sharing functionality between classes.
A follow-up question can make this more challenging:
When would you use a trait instead of inheritance?
The interviewer is looking for whether the candidate understands the design decision, rather than simply remembering the syntax.
Question 5: How does exception handling work in PHP?
An experienced candidate should be comfortable with:
trycatchfinallythrow
They should understand the difference between handling an expected application error and simply hiding an error.
Question 6: What are type declarations in PHP?
A three-year developer should be familiar with type declarations for parameters, return values and properties.
The interviewer can ask followup question like this –
Why are type declarations useful in a large application?
This tests whether the candidate understands how types can make code easier to maintain and catch errors earlier.
2. Object-Oriented PHP Interview Questions
Object-oriented programming is an important part of most enterprise PHP applications.
At three years of experience, candidates should be comfortable working with classes and should understand the design principles behind them.
Question 7: What is the difference between an interface and an abstract class?
This is a very common question, but the useful part is the discussion and followup questions –
Interviewer: If you have multiple payment providers that must implement the same operations, would you use an interface or an abstract class? Why?
The candidate should be able to explain with examples the differences. Interviewer is not just looking for definitions. Use real world examples to explain the difference.
Question 8: What is dependency injection?
A candidate should understand that dependencies can be provided to a class instead of the class creating those dependencies itself.
For example, instead of a service creating its own database or payment client, the required dependency can be supplied to it.
A followup question can be:
Why does dependency injection make testing easier?
Question 9: What is polymorphism?
The candidate should be able to explain polymorphism using a practical example, preferably something they have encountered in application development.
Question 10: What is the difference between composition and inheritance?
This is a good question for a three-year developer because it moves beyond basic OOP definitions.
The candidate should understand that inheritance creates an “is-a” relationship, while composition allows a class to use other objects to provide functionality.
Question 11: What are the SOLID principles?
Interviewer is not looking for a text book definition of the principle. An experienced developer should try to explain with strong examples with concept which can answer question like this –
What problem does the Single Responsibility Principle try to prevent?
Answering this will reveal if the candidate really understands the principle and worked with maintainable code. This is better than just a definition.
3. Database and SQL Interview Questions
PHP applications commonly depend heavily on relational databases, so SQL will be an important part of a three-year experience PHP developer interview.
Question 12: What is the difference between INNER JOIN and LEFT JOIN?
The candidate should be able to explain what happens when matching records do not exist.
A small SQL query can be mentioned to explain the concept and talk about what happens when records exist vs non-existing records. Basically try to understand if candidate only know the definitions or understands the usage.
Question 13: What is a database index and when would you use one?
Experienced developer should know how indexes can make queries fast. The answer should also touch on what happens with storage and how additional storage is required when we increase the indexes.
Question 14: How would you investigate a slow SQL query?
A good answer should talk about multiple points –
- Looking at the query
- Looking for missing or ineffective indexes
- Checking joins and filters
- Checking whether the application is making unnecessary queries
Question 15: What is a database transaction?
The candidate should understand why transactions are useful when multiple database operations need to succeed or fail together.
A follow-up question from interviewer can be like –
Suppose you are transferring money between two accounts. Why would you use a transaction?
Question 16: What is the N+1 query problem?
This is particularly useful when interviewing developers who have worked with ORMs.
The candidate should be able to recognize a situation where an application makes one query to retrieve a list and then makes another query for each item in that list.
They should also be able to suggest ways to avoid it.
4. API and Web Development Questions
A PHP developer with 3 years experience may have built or used APIs, so basic web concepts can be asked –
Question 17: What happens when a browser sends an HTTP request to a PHP application?
This is a common question that can be asked to understand how candidate understand entire application stack.
Experienced candidate should be able to discuss, at an appropriate level, the request, web server, PHP application, processing, database interaction and HTTP response.
Question 18: What is the difference between authentication and authorization?
Authentication answers: Who are you?
Authorization answers: What are you allowed to do?
The candidate should understand the difference between these two and give a practical example to explain any simple application. Real world application examples work better while explaining the concepts.
Question 19: What HTTP status codes would you commonly use in a REST API?
The candidate should be familiar with common codes such as:
200201400401403404422500
The important thing is understanding when to use them, not memorizing a list. Know the status code meaning and at what conditions do these status code are returned. Experienced candidates can give examples of when these are returned and what should the next action be for an application.
Question 20: How would you handle a third-party API that fails?
This is a good scenario-based question.
Look for discussion around:
- Timeouts
- Error handling
- Retries
- Logging
- Appropriate user responses
- Avoiding duplicate operations
- Monitoring failures
The exact implementation will depend on the API and application, but the candidate should explain and should talk about failures that can happen when APIs don’t work as expected.