When it comes to developing with WordPress, understanding the core PHP functions is essential for efficient and effective coding. This knowledge not only enhances your ability to create custom themes and plugins but also optimizes the overall performance of your WordPress sites. Here are the Top 5 PHP Functions every WordPress developer should know:

1. get_option(): This function retrieves the value of a specific option from the database, making it crucial for theme and plugin development.
2. update_option(): Used to update an existing option in the database, allowing developers to change settings dynamically.
3. add_action(): A vital function for adding custom actions to WordPress hooks, essential for modifying default behaviors.
4. add_filter(): Similar to add_action(), this allows developers to filter data before it is processed, providing great control over output.
5. wp_enqueue_script(): This function is used to safely add JavaScript files to your WordPress site, ensuring proper loading and preventing conflicts.

Customizing your WordPress site using PHP can seem daunting at first, but with the right approach, you'll find that it empowers you to tailor your site's functionality to suit your needs. To begin, ensure you have a child theme activated to prevent any modifications from being overwritten during updates. Start by accessing your theme's functions.php file, where you can insert your custom PHP code. Here’s a simple step-by-step guide to get you moving:

1. Open your functions.php file from the WordPress admin panel.
2. Add your custom function, making sure to use unique names to prevent conflicts.
3. Use the add_action or add_filter functions to integrate your code with WordPress's event-driven architecture.

Once you've added your custom PHP functions, the next step is to test them thoroughly. You can do this by visiting the front end of your site to see if the modifications are visible and functioning correctly. Remember, debugging is a critical part of PHP development, so utilize tools like WP_DEBUG to pinpoint errors. Finally, always keep your WordPress core, themes, and plugins up-to-date to ensure compatibility. Customizing your WordPress site using PHP not only enhances its functionality but also gives you a unique edge, making your site stand out from the crowd.

When it comes to PHP development in WordPress, following best practices is essential for ensuring optimal performance and maintainability. First and foremost, developers should adhere to the WordPress Coding Standards, which provide guidelines for writing clear and consistent code. This includes utilizing proper indentation, naming conventions, and file organization. Additionally, leveraging WordPress hooks, such as actions and filters, allows for modularity and reusability of code, making it easier to troubleshoot and update in the future. Another critical practice is to ensure proper sanitization and validation of user input to enhance security.

Moreover, utilizing modern PHP features can greatly enhance the quality of your code. Incorporating object-oriented programming (OOP) principles helps in creating reusable components and improves the overall structure of your application. Furthermore, adhering to the SOLID principles can lead to better scalability and easier maintenance. It's also crucial to make use of version control, such as Git, to track changes and collaborate with others efficiently. Finally, regularly updating your PHP version and WordPress core can mitigate potential security vulnerabilities and improve performance.