Mastering PHP is crucial for any developer looking to build dynamic web applications. There are certain PHP functions that stand out due to their versatility and importance. Here are 10 essential PHP functions that every developer should master:

1. print() - Used for outputting strings to the screen, it's one of the most basic functions.
2. array_merge() - This function allows you to combine two or more arrays, which is invaluable when handling multiple datasets.
3. explode() - It splits a string by a specified delimiter, turning it into an array, which is essential for data manipulation.

Continuing with our list, we also see some functions that enhance the functionality of PHP:

4. implode() - The opposite of explode(), it joins array elements into a single string.
5. json_encode() - This function converts PHP arrays and objects into JSON format, widely used for APIs.
6. curl_init() - A powerful function for making HTTP requests, vital for integrating with other web services or APIs.
7. isset() - Checks if a variable is set and is not null, which helps avoid unexpected errors.
8. header() - Sends raw HTTP headers, crucial for controlling your website’s response.
9. die() - Terminates the current script; while seemingly harsh, it can be very useful in controlling script flow during error handling.

When working with PHP, developers often encounter common pitfalls that can lead to significant time loss and frustration. One of the main mistakes is neglecting to validate user input, which can result in security vulnerabilities and unexpected behaviors. Always ensure that user data is sanitized and validated through functions like filter_input() and htmlspecialchars(). Additionally, failing to handle errors gracefully can lead to a poor user experience. Implementing proper error handling via try-catch blocks can save not just time but also aid in maintaining cleaner code.

Another frequent issue arises from not understanding the importance of variable scope in PHP. Many beginners mistakenly assume that a variable is accessible throughout the entire script, which can lead to uninitialized variables and unexpected results. To avoid this, it's crucial to be mindful of where your variables are declared and to use global variables only when absolutely necessary. Furthermore, developers should refrain from using deprecated functions, as these can lead to compatibility issues in future versions of PHP. Keeping your code up-to-date ensures smoother transitions in the long run.

Writing clean and efficient PHP code is essential for maintaining high performance and readability in your projects. First and foremost, consistency is key. This includes adhering to a naming convention for functions and variables, utilizing proper indentation, and organizing your code into logical sections. Additionally, consider using comments effectively to explain the purpose of complex code blocks. This practice not only aids in the initial development phase but also benefits anyone who might work on the code in the future, improving overall collaboration.

Another best practice is to leverage built-in PHP functions and libraries whenever possible. This approach reduces the need to reinvent the wheel and can significantly enhance the efficiency of your code. Furthermore, employing object-oriented programming (OOP) principles can lead to more modular and reusable code, which simplifies maintenance. Implementing error handling and logging mechanisms also ensures that your application can gracefully handle exceptions, leading to a better user experience. Emphasizing readability and efficiency from the outset will pay dividends as your project scales.