Key Principles and Practical Tips for Best PHP Code Reviews

Estimated read time 11 min read


Code reviews can sometimes feel a bit tedious. But they are absolutely necessary to create PHP applications that work well, are easy to use, and don’t cause security issues. The good news? There is a way to make code reviews effective. Let’s break down the key things to look for when reviewing PHP code.

core principles

1. Functional Check: Does the code do its job?

The most important aspect of code review is ensuring that the code achieves its intended purpose. Focus on the code logic, the execution flow from receiving input to producing output. Check for illogical steps, incorrect calculations, or places where the process might stop unexpectedly.

  • Check input : Does the code correctly handle all types of data it may receive? This includes user input, database data, or information from external systems.
  • Check the output : Verify that the results produced by the code are correct and in the expected format. Does the output data meet the requirements?

Thorough testing is key to ensuring functionality. Unit testing helps us systematically check various components of the code with different input variables, ensuring that the code runs as expected in all situations.

During this step, I find it helpful to be able to publish the code to a review application or staging server and confirm what I discovered in the code review and how it actually works. For the tricky parts, I also tend to search for added unit tests. If they are missing, it might be a good idea to ask the author to add them.

2. Code functionality: Does it work as designed?

At the heart of a solid code review, we need to answer a fundamental question: Does the code do what it’s supposed to do? Start directly comparing the code to the project’s requirements or specifications. Have you implemented all necessary functionality? Is there incorrect behavior or anything missing? Next, carefully step through the logic of the code. Does execution follow a reasonable path from received input to final output? Look for any meaningless branches (such as if statements that are always false), infinite loops, or potential crashes.

Check how the code handles all forms of input. Can it handle different user entries, various data pulled from a database, or information from another system? Just as important, is the output correct, well-formatted, and consistent with what is expected from other parts of the application?

Tech Tip: Don’t just test by clicking on an app. While developers bear the primary responsibility for writing unit tests, don’t underestimate the value of a critical eye during code reviews.

  • Missing tests: Are there blocks of code that don’t have corresponding unit tests?
  • Edge use cases: Do the tests only cover expected scenarios, or do they include unexpected inputs and boundary conditions?
  • Test quality: Are the tests well written and do they clearly state the expected results?

As you inspect, imagine ways in which a user might intentionally (or accidentally) try to break the code. Can you feed it strange inputs, cause unusual sequences of events, or overload it? Resilient code should handle these scenarios gracefully. Proficient in using debugging tools like Xdebug. It allows you to pause code execution, step through line by line, and double-check the values ​​of variables when things change. For front-end code, I like to think about the different UI states that may occur.

Some key states include empty state, loading state, and error state, but it’s important to go further:

  • Partially loading status: How does data appear when loading progressively? Are there clear loading indicators for different parts of the UI?
  • Input validation status: How does the UI immediately communicate success or failure of form validation (e.g., inline error message)?
  • Success status: After an action (e.g., submitting a form), how is success communicated?
  • Interaction state: Does the element provide visual feedback of hover, focus, or activity status?

3. Code readability: Can you read it?

Readable code is essential for maintainability and collaboration. Let’s focus on making your code easy for humans and machines to parse. Start by strictly adhering to coding standards like PSR-1 and PSR-12. These standards establish a common language for PHP code, defining rules for indentation, naming conventions, file organization, and more.

By following standards, your code becomes predictable and consistent, reducing the cognitive load on readers. Community standards like PSR minimize the learning curve for developers new to the project and improve compatibility with different development tools.

During code review, carefully evaluate variable and function naming. Do the names clearly express their purpose, avoiding single-letter variables, unnecessary abbreviations, or vague terms? Well-named elements contribute to self-documenting code, minimizing the need for explanatory comments. If there are comments, are they focused on explaining the “why” behind the logic or design choices, rather than simply repeating what the code does?

If the code feels complex, the author is advised to refactor it. This might involve extracting methods, using more descriptive variable names, or restructuring blocks of code for clarity. Emphasize the importance of long-term maintainability, even if it currently requires some extra effort.

Use linkers (such as PHPCS) and static analysis tools (such as PHPStan) as part of the review process. These tools help enforce standards, catch potential problems, and promote consistent readability. Look for mismatches between code and established standards as potential areas for improvement.

If you find yourself struggling to understand the code flow during a review, this is a strong indication that maintainability will be a challenge in the future. Don’t hesitate to raise this point with the author – collaborative discussion can often uncover better solutions or clarify the underlying logic.

In addition to formatting and naming, strictly adhere to project or company-specific coding rules. These cover aspects such as namespaces, code organization, and architectural patterns. While automated tools can catch many violations, remain vigilant during the review process to uncover potential issues that the tool may miss. This ensures consistency across the entire codebase.

4. Security

Web applications are prime targets for attacks. In the PHP world, secure code reviews focus specifically on a few key areas.

First, never trust data from external sources Handle all user input (form submissions, URL parameters, etc.) that is potentially malicious Use PHP’s built-in filter functions ( filter_varfilter_input) to remove dangerous characters (e.g. <script>tags to prevent XSS) and enforce rules to Make sure the input matches what you expect (for example, a correct email format or a valid number range).

To protect your application from the notorious SQL injection vulnerabilities, avoid concatenating user input directly into SQL queries. Instead, rely on mysqli or PDO prepared statements (or better yet, on a Database Abstraction Layer [DBAL] or some good ORM). They clearly separate SQL structures from user-supplied data, allowing the database to safely handle data and eliminate SQL injection attempts. Remove dangerous characters (e.g. <script> tags to prevent XSS) and enforce rules to ensure input matches your expectations.

Finally, handle errors with care. Avoid displaying raw error messages (database errors, stack traces) to the user as they may reveal sensitive system information. Instead, log errors to a file for developers to troubleshoot, ensuring the logs themselves are protected from unauthorized access. When an error occurs, display a generic, helpful error message to the user and log details for internal debugging. In our case we mainly use Monologand forward logs to tools such DataDogas NewRelic. We also always have a sentinel instance connected to collect more information on issues.

While modern frameworks offer built-in security features, it is crucial to ensure they are implemented correctly during code review. Pay close attention to these areas:

  • Input sanitization: Does the code carefully filter and validate any data coming from the user (forms, URL parameters, etc.)? Look for a framework-specific input sanitization function or method.
  • Prepared statements: Are database queries always built using prepared statements? Check framework methods that help prevent SQL injection.
  • Error handling: Does the code avoid exposing raw error messages or stack traces to the user? Are errors logged internally so developers can troubleshoot? Are there user-friendly fallback mechanisms in place in the event of a failure?

5. PHP performance optimization

Slowly executing code can frustrate users and potentially drain server resources. A comprehensive code review should always consider performance optimization, focusing specifically on the following areas:

  • Smarter algorithms : The way you structure your code has a big impact on speed. Analyze your core algorithm and look for opportunities to use more efficient data structures (for example, consider hash tables instead of nested loops for searches). Being familiar with Big O notation helps understand how the efficiency of your code scales with larger data sets.
  • Database interaction : Every query to the database adds overhead. Reduce unnecessary database calls by using caching technology (Memcached, Redis) to store frequently accessed data in memory. When you do need to query, optimize your SQL: use indexes appropriately, avoid fetching more data than you need, and be aware that complex joins can slow things down. Before you start using caching, focus first on indexing and query optimization.
  • Find bottlenecks : don’t optimize blindly! Use an analytics tool like Blackfire to measure exactly where your application spends the most time. This will pinpoint the functions or database queries that require the most attention. Blackfire provides valuable insights into execution times, function calls, and memory usage.

Technical Description

  • Premature optimization is a trap: Focus on clean, functional code first. Over-optimizing prematurely can make your code harder to read.
  • Caching comes in many forms: choose between in-memory caching, file-based caching, or even HTTP caching, depending on your application’s needs.
  • The larger the data set, the greater the impact of the algorithm: code that works well on small-scale data may crawl as the input size increases.

Please pay special attention to database migration. Pay close attention to database migrations, taking into account both code performance and the migration process itself. Large migrations can take quite a while (maybe even minutes), so it’s crucial to understand the potential impact in advance.

Code review considerations

While a thorough dependency audit goes beyond a typical code review, here are the key things to note:

  • Severely outdated packages: Note any major version differences between an installed package and its latest version. This could mean potential compatibility issues or security risks.
  • Vulnerability Alerts: If you use tools like Snyk or Dependabot, check if they have flagged any known vulnerabilities in your project’s dependencies.
  • Version meaning: It is recommended that when updating packages, please pay attention to semantic versioning (major.minor.patch), because major updates may have breaking changes.
  • Company Standards: Some organizations have specific policies regarding dependency updates, and reviewers should be familiar with these policies.
  • Scope of review: If time permits or security is a critical issue, a brief dependency scan using a verifiability checking tool can be a valuable addition to code review.

Database optimization and security

Always ensure that database queries use prepared statements consistently to mitigate SQL injection risks. Use analysis tools integrated with your database (e.g., MySQL’s Slow Query Log) or extensions like Blackfire/New Relic to identify the most effective optimizations. Pay Attention to Indexes – Make sure there are appropriate indexes on frequently queried columns, especially for tables with multi-column search criteria.

Error handling

Define a custom exception class that creates an error hierarchy (for example: DatabaseExceptionValidationException). This approach enables granular error handling throughout the code base. Use different log levels (debug, info, warning, error) strategically.

Configure logging tools appropriately to store or alert based on severity. Finally, carefully consider the error messages displayed to users. These messages should clearly guide users to the solution without revealing sensitive system details.

User-reported cybersecurity issues can be tricky due to limited information. That’s why clear error messages and detailed logs are essential. They give you the clues you need to quickly pinpoint problems and improve user experience.

Make code review a habit

Code reviews should not be viewed as a one-time chore, nor as a way to simply find bugs. By making them a regular practice during development, you will continually improve the quality of your PHP codebase. Each review meeting builds technical knowledge and enhances collaboration within the team.

Code reviews are also a great way to transfer knowledge among your team. Not only does the developer who writes the task know how it is implemented, but the person doing the code review will also have a good understanding of it. In our case, we ensure that every line added, removed, or changed is reviewed by at least one other person.

Remember, clean, secure, and well-structured code is more than just aesthetics. It saves debugging time, reduces the risk of vulnerabilities, and allows your application to scale more gracefully. Make code reviews a non-negotiable part of your workflow. Over time, these benefits will compound, resulting in more robust, easier to maintain, and more successful PHP projects.

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