Failing a PHP Interview: 3 Typical Mistakes and How to Avoid Them

PHP Interview: Where to Start to Avoid Failure?

You sent your resume, had a short call with HR, and now — an invitation to a technical interview. Your heart starts beating faster: "What if they ask about something I don't know?", "What if I forget the syntax?", "What if they ask me to write code on a whiteboard?"

Spoiler: they probably will. And that's normal. A PHP interview is not an exam on tickets, but a dialogue. But a dialogue you need to prepare for. In this article, we'll break down what PHP developers are actually asked in interviews in 2025, what mistakes 90% of candidates make, and how to present yourself in the best light.

What's Important to Know: Preparation Structure

To make your PHP interview preparation systematic, break it down into three blocks:

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• Language Basics: data types, arrays, functions, file handling, sessions.
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• OOP and Architecture: inheritance, encapsulation, polymorphism, traits, interfaces.
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• Tools and Ecosystem: Composer, PSR, frameworks, testing, SQL.
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One of the most common questions in a PHP junior interview is the difference between == and ===. It seems like a small thing, but it's on these small things that even experienced developers stumble.

Example: Comparison with Type Juggling

<?php\n// Loose comparison\nvar_dump(0 == '0'); // true\nvar_dump(0 == false); // true\nvar_dump('' == false); // true\n\n// Strict comparison\nvar_dump(0 === '0'); // false\nvar_dump(0 === false); // false\nvar_dump('' === false); // false\n?>

Why is this important? Because in real projects, loose comparison can lead to hard-to-find bugs. For example, when checking for a value in an array using in_array() without the third parameter true.

OOP in PHP: What is Asked Most Often

In a PHP interview for middle and senior levels, OOP questions are mandatory. But juniors should also be prepared. Here are the top 3 topics:

1. Traits vs Interfaces vs Abstract Classes

Many people confuse when to use a trait versus an interface. Remember a simple rule:

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• Interface — is a contract. The class commits to implementing the methods.
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• Abstract Class — partial implementation + contract.
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• Trait — is simply a set of methods that can be "pasted" into any class.
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<?php\ninterface LoggerInterface {\n    public function log(string $message): void;\n}\n\nabstract class AbstractLogger implements LoggerInterface {\n    protected string $prefix;\n\n    public function __construct(string $prefix) {\n        $this->prefix = $prefix;\n    }\n\n    abstract public function formatMessage(string $message): string;\n}\n\ntrait TimestampTrait {\n    public function getTimestamp(): string {\n        return date('Y-m-d H:i:s');\n    }\n}\n\nclass FileLogger extends AbstractLogger {\n    use TimestampTrait;\n\n    public function formatMessage(string $message): string {\n        return $this->getTimestamp() . ' [' . $this->prefix . '] ' . $message;\n    }\n\n    public function log(string $message): void {\n        file_put_contents('log.txt', $this->formatMessage($message) . PHP_EOL, FILE_APPEND);\n    }\n}\n?>

2. SOLID Principles

This is not just an acronym. Interviewers love to ask: "Give an example of a violation of the Liskov Substitution Principle (LSP)". The most illustrative example is the classic "square inherits from rectangle".

<?php\n// LSP Violation\nclass Rectangle {\n    public function __construct(protected int $width, protected int $height) {}\n    public function setWidth(int $width): void { $this->width = $width; }\n    public function setHeight(int $height): void { $this->height = $height; }\n    public function getArea(): int { return $this