Mastering PHP is essential for any developer aiming to create robust web applications. Here's a list of the Top 10 PHP Functions every developer should master to enhance their coding efficiency and application performance. These functions not only simplify the coding process but also provide powerful features to handle data, manipulate strings, and manage arrays effectively. Understanding these functions will significantly streamline your development workflow and improve code readability.

1. strlen(): This function is crucial for determining the length of a string. Knowing the exact length can help in validation and formatting tasks.
2. array_push(): As a developer, you often need to add elements to arrays. This function allows you to efficiently add one or more elements to the end of an array.
3. explode(): This function is invaluable for breaking strings into arrays based on a specified delimiter, simplifying data manipulation.
4. implode(): Conversely, implode allows you to join array elements into a single string, making data presentation easier.
5. json_encode(): In today’s API-driven world, converting data into JSON format is vital for client-server communication.
6. json_decode(): This function complements json_encode by converting JSON strings back into PHP arrays or objects.
7. file_get_contents(): For fetching remote files or data, this function provides a simple solution, making it easier to work with external content.
8. isset(): Essential for checking if a variable is set and not null, ensuring error-free code execution.
9. array_map(): This function allows developers to apply a callback function to each element of an array, streamlining transformations and modifications.
10. header(): Critical for sending raw HTTP headers, this function plays a vital role in controlling HTTP responses.

Debugging is an essential skill for any PHP developer. In this ultimate guide, we will explore various PHP debugging techniques that can help streamline your development process and enhance your productivity. From utilizing built-in functions to employing powerful debugging tools, you'll be equipped with the knowledge to identify and fix issues efficiently. Here are some key techniques to kick off your debugging journey:

1. Var_dump(): This function displays structured information (type and value) about one or more variables. It is particularly useful for examining large arrays and objects.
2. Error Reporting: Always ensure your error reporting is set to a level that will notify you of all issues. Use error_reporting(E_ALL); to show all types of errors during development.
3. Xdebug: An essential tool that provides stack traces and function traces in error messages, as well as full variable dumps. Xdebug helps in visualizing code execution and catching issues early.

By mastering these techniques, you'll be able to tackle any PHP challenge that comes your way, making your applications more robust and reliable.

Optimizing your PHP code is crucial for enhancing application performance and user experience. To begin with, consider employing opcode caching. Tools like OPcache can significantly reduce script execution times by storing precompiled script bytecode in memory. Additionally, avoid using unnecessary variables and data structures; instead, utilize built-in functions that are optimized for performance. Another tip is to minimize I/O operations, as they can be time-consuming. For instance, retrieving data in batches rather than one by one can greatly improve your application's speed.

Furthermore, always ensure efficient database interactions when using PHP. Implementing indexes on database tables can lead to faster query performance. When possible, use prepared statements to enhance security and performance while interacting with a database. Moreover, adopting a strict coding standard helps in maintaining code quality, making it easier to identify performance bottlenecks. Finally, periodically profiling your code with tools like Xdebug or Blackfire can provide valuable insights into where optimizations should be focused, ensuring your PHP application runs at optimal speed.