When diving into PHP development, understanding the core functions is crucial for every developer's toolkit. Here are 10 essential PHP functions that can significantly enhance your coding efficiency:

1. strlen() - This function is used to get the length of a string, helping developers manage text processing effectively.
2. empty() - A vital function that checks whether a variable is empty or not, which is essential for validating user inputs.
3. array_push() - Appends one or more elements to the end of an array, a common operation in array manipulation.
4. json_encode() - Converts a PHP array or object to a JSON string, making it easier to work with APIs.
5. explode() - This function splits a string by a specified delimiter, useful for parsing data.
6. implode() - The counterpart to explode(), it joins array elements into a string, often used for creating query strings.
7. header() - Sends raw HTTP headers, essential for redirecting users or controlling cache settings.
8. isset() - Checks if a variable is set and is not null, crucial for avoiding errors in script execution.
9. substr() - Returns a part of a string, very handy for string manipulation tasks.
10. date() - Formats a local date and time, a must-know for any application dealing with time-sensitive data.

When looking to optimize your PHP code for performance, one of the first steps is to profile your application. Use tools such as Xdebug or Blackfire to identify bottlenecks in your code. Analyzing the output will help you pinpoint slow-running functions and memory-intensive operations. Once you have this information, you can focus on optimizing the most problematic sections. Consider using caching mechanisms like OPcache to store precompiled script bytecode in memory, which reduces the amount of time spent on parsing and compiling PHP scripts on each request.

Another effective strategy is to minimize the use of expensive operations. For instance, avoid using foreach loops for large datasets when functions like array_map or array_filter can be utilized instead. Additionally, reduce the frequency of database queries by implementing batch processing or using prepared statements. Remember, the goal is not just to write functional code but to ensure it runs smoothly and efficiently. By adopting these practices, you can significantly enhance the performance of your PHP code, leading to faster response times and better user experiences.

When it comes to secure PHP development, adhering to best practices is paramount. First and foremost, always validate user inputs to prevent SQL injection and other malicious attacks. Utilize parameterized queries or prepared statements when dealing with database operations. Moreover, implementing output encoding, such as HTML entity encoding, can help mitigate cross-site scripting (XSS) vulnerabilities. Here are some essential practices for secure PHP development:

1. Use secure connections: Always employ HTTPS to encrypt data between the client and server.
2. Keep PHP and all libraries updated: Regularly update your PHP version and all third-party libraries to protect against known vulnerabilities.
3. Practice the principle of least privilege: Limit user permissions and provide access only to necessary features.

Additionally, consider utilizing security-focused tools and frameworks that can help bolster your PHP applications. Implementing Content Security Policy (CSP) can significantly increase protection against XSS attacks by defining which sources are permitted to load content. Secure PHP development also involves sanitizing all outputs, not just user inputs, to avoid exposure of sensitive data. Lastly, regularly conduct security audits and code reviews to identify potential vulnerabilities. By fostering a security-first mindset, you ensure that your PHP applications remain robust against various cyber threats.