When it comes to PHP development, adhering to best practices is crucial for ensuring code quality and maintainability. The first established practice is to consistently follow the PSR standards, such as PSR-1 and PSR-2, which outline coding style guidelines. This helps to standardize code, making it easier for developers to read and collaborate. Next, employing a version control system, like Git, allows teams to manage changes efficiently and roll back if issues arise, enhancing overall code quality.

Thirdly, implementing unit testing is vital for catching bugs early in the development process. By writing tests for each component, developers can ensure that changes do not introduce new errors. Additionally, using dependency injection can improve the modularity of your code, making it easier to manage and test. Lastly, always prioritize code documentation; documenting your code not only aids in future maintenance but also helps new team members understand the project quickly.

When starting with PHP, beginners often encounter common mistakes that can lead to frustrating errors and inefficient code. One significant error is not properly understanding variable scope. Failing to grasp variable visibility can result in unexpected behavior, especially when working with functions. To avoid this, it's essential to use global and local scopes correctly. Additionally, beginners frequently overlook the importance of error reporting; ensuring that error reporting is enabled in your PHP configuration can help identify issues early. This includes using error_reporting(E_ALL); to capture all types of errors.

Another frequent mistake among newcomers is neglecting proper validation and sanitization of user inputs. Failing to validate and sanitize inputs can lead to security vulnerabilities, such as SQL injection attacks. Beginners should always employ functions like filter_var() and prepared statements when dealing with user data. Additionally, not utilizing comments effectively can lead to confusion later on, especially in larger projects. Keeping your code well-commented makes it easier to understand and maintain. Therefore, cultivate the habit of writing clear, concise comments to elucidate your code's purpose.

Optimizing your PHP code for better performance is essential for enhancing the overall speed and efficiency of your web applications. Start by utilizing caching mechanisms such as opcode caching with tools like OPcache, which stores precompiled script bytecode in memory, greatly reducing execution time. Additionally, consider employing content delivery networks (CDNs) to distribute static resources, resulting in faster load times for users across various locations.

Another key strategy for performance optimization is to minimize database queries. This can be achieved by using indexed columns and appropriately structuring your database architecture. Moreover, always validate and sanitize input data to avoid unnecessary processing overhead. You can further enhance performance by leveraging built-in PHP functions instead of writing custom implementations, as these functions are typically fine-tuned for speed. Remember, well-optimized PHP code translates to a better user experience and increased site efficiency.